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Shell keep
Shell keep
from Wikipedia
An aerial photograph of Windsor Castle, with three walled areas clearly visible, stretching left to right. Straight roads stretch away in the bottom right of the photograph, and a built-up urban area can be seen outside the castle on the left.
An aerial view of Windsor Castle with its shell keep (called "The Round Tower") prominent on its motte inside the middle ward (middle bailey)

A shell keep is a style of medieval fortification, best described as a stone structure circling the top of a motte.

In English castle morphology, shell keeps are perceived as the successors to motte-and-bailey castles, with the wooden fence around the top of the motte replaced by a stone wall. Castle engineers during the Norman period did not trust the motte to support the enormous weight of a stone keep. A common solution to this problem was to replace the palisade with a stone wall and then build wooden buildings backing onto the inside of the wall. This construction was lighter than a keep and prevented the walls from being undermined, meaning they could be thinner and lighter.

Examples

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A gazetteer compiled by archaeologist Robert Higham counted 21 shell keeps in England and Wales.[1] Examples include the Round Tower at Windsor Castle[2][3] and the majority were built in the 11th and 12th centuries.[4]

Surviving English examples of shell keeps include:

  • Arundel, West Sussex (re-modelled post-medieval)
  • Berkhamsted, Hertfordshire
  • Carisbrooke, Isle of Wight
  • Castle Acre, Norfolk (shell keep around an inner tower or manor House)
  • Clare, Suffolk (part of wall on motte only)
  • Fotheringhay, Northamptonshire (demolished - motte only survives)
  • Launceston, Cornwall
  • Lewes, East Sussex – two shell keeps on same site? One survives.
  • Lincoln, Lincolnshire – two shell keeps on same site? One survives.
  • Oxford, Oxfordshire
  • Pickering, North Yorkshire
  • Restormel, Cornwall (excellent example)
  • Tamworth, Staffordshire
  • Tonbridge, Kent (foundations on motte only)
  • Totnes, Devon
  • Trematon, Cornwall
  • Warwick, shell demolished and incorporated into bailey wall post-medieval
  • Wigmore, Herefordshire
  • Windsor, Berkshire (re-modelled post-medieval)
  • Wiston (Wales)

In addition Farnham and Berkeley castles have stone enclosed mottes which may be interpreted as shell keep variations. At other sites such as Durham, Warkworth, Clifford's Tower (York) and Sandal (Wakefield), shell keeps may have evolved into a tower proper. Clifford's Tower is often interpreted by modern visitors as a shell keep due to explosion damage, in 1684, which removed the roof and its central supporting masonry. True shell keeps were a stone wall around the upper perimeter of the motte with lean-to buildings against this outer wall and a small courtyard in the middle. See Restormel plan below.

Unusually Lewes and Lincoln castles both feature two separate mottes which may have had shell keeps upon both of them. The reason for this is unclear but given that Lincoln Castle is adjacent to the cathedral, one shell keep may have been for the castellan (castle holder) and the other for the bishop. In each case, only one shell keep survives.

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Citations

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General and cited references

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from Grokipedia
A shell keep is a type of medieval fortified enclosure, typically circular or polygonal in form, constructed from stone atop the artificial mound (motte) of an earlier , serving as a defensive curtain wall that enclosed a central open space for buildings and activities. This design replaced the vulnerable wooden palisades of Norman-era castles with a more durable stone shell, providing protection against fire while utilizing the existing motte for elevation and defense. Primarily developed in during the 12th and 13th centuries, shell keeps represented a transitional between timber fortifications and more advanced stone tower keeps, often built by wealthy lords to upgrade existing sites amid ongoing threats like or Welsh campaigns. Shell keeps emerged in the early as a practical response to the limitations of wooden structures, which were prone to destruction by siege engines or incendiary attacks, and their construction accelerated after the to consolidate control over conquered territories. By the mid-, they became a common retrofit for mottes, with around 70 examples surviving in the today, though their popularity waned in the 13th century as rectangular great towers and concentric designs offered superior strength and space. Historians note that shell keeps were not uniform; some incorporated multi-lobed walls for added stability, and their development reflects broader shifts in military architecture toward masonry without requiring entirely new foundations. Architecturally, shell keeps featured thin yet robust walls—typically 2 to 3 meters thick and 6 to 10 meters high—to minimize load on the motte and prevent , enclosing an area of 25 to 50 meters in diameter with a simple gateway for access. Internal arrangements varied, but commonly included wooden halls, chapels, and chambers built against the inner face of the wall, facing a central that allowed for and ventilation while maintaining defensibility. Unlike solid tower keeps, the shell design prioritized enclosure over a single massive structure, functioning as both a residence for the and a final refuge during sieges, often integrated into larger complexes with outer baileys. Notable examples include Restormel Castle in , the best-preserved shell keep with its near-complete 8-meter-high walls dating to the early 13th century, and Clifford's Tower in , a unique quatrefoil-shaped structure built in the mid-13th century. Other significant sites are Windsor Castle's motte-top enclosure from the 12th century, which influenced royal fortifications, and Tamworth Castle in , featuring internal ranges around a courtyard. These remnants highlight the shell keep's role in medieval defensive strategy, blending economy with efficacy, and continue to inform archaeological studies of castle evolution.

History

Origins in Motte-and-Bailey Castles

The , an earth-and-timber fortification consisting of a raised mound (motte) topped by a wooden structure and an adjacent enclosed courtyard (bailey), was introduced to by the following their conquest in 1066. These structures were rapidly erected to assert control over newly conquered territories, with ordering the construction of wooden castles at key sites immediately after the , such as at in 1068 and . By around 1100, the had built approximately 1,000 such castles across to secure their rule and suppress rebellions. The design originated in during the early , where motte-and-bailey prototypes evolved from earlier 10th-century earthen fortifications, influencing their adoption in for swift deployment during the conquest period of 1066 to the 1080s. On the motte, a wooden formed a defensive , often surrounding a central tower or hall that housed the lord, his family, and a , while the bailey accommodated stables, workshops, and additional troops. Examples like , one of the earliest post-conquest sites built around 1070, exemplify this layout, with its motte providing elevated defense and the bailey supporting logistical needs. The vulnerability of these wooden elements to and warfare—demonstrated in attacks where flames could rapidly destroy palisades and structures—prompted experimentation with stone by the late . This shift addressed the limitations of timber, which proved inadequate against prolonged assaults, laying the groundwork for more durable enclosures like shell keeps on existing mottes.

Evolution and Peak Construction

The shell keep emerged as a transitional form of in the mid-, evolving from the wooden palisades atop motte-and-bailey structures to provide greater durability against fire and siege tactics. This development was particularly accelerated during (1135–1153), a period marked by widespread instability that prompted rapid upgrades for security, as seen in sites like where a stone shell keep was constructed by the early to withstand sieges. Construction peaked in the mid-to-late , with approximately 70 known examples across built primarily between 1150 and 1216, coinciding with the reign of Henry II (1154–1189) and extending into those of his successors. Royal initiatives under Henry II drove significant investments in stone fortifications, including shell keeps at royal castles, while marcher lords in adopted the design to secure border regions against Welsh resistance, exemplified by early adopters like , where the circular shell keep was erected around 1150–1153 by Robert Fitzharding. By the early , shell keeps began to decline as military advanced toward more sophisticated concentric designs, which offered layered defenses without relying on artificial mottes, particularly under Edward I's campaigns in from the 1270s onward. The introduction of in the late 13th and 14th centuries further rendered elevated shell keeps vulnerable to , shifting focus to lower, more robust enclosures.

Architectural Features

Structure and Components

A shell keep consists of thick, curved stone walls typically measuring 3 to 3.5 meters (10 to 12 feet) in thickness and 4.5 to 9 meters (15 to 30 feet) in height, forming a hollow enclosure that encircles the summit of a motte and surrounds an open central courtyard. These walls create a circular or polygonal shell, providing a lightweight yet robust defensive perimeter adapted from earlier wooden palisades. Access to the interior is gained through a single entrance, often featuring a or a steep ramp to enhance by limiting approach points. The internal diameter of the generally ranges from 25 to 50 meters, allowing space for activities while maintaining structural integrity on the elevated motte. The walls are constructed directly along the edge of the motte, leveraging the mound's natural height for added defensive advantage without overburdening the earthwork foundation. This integration exploits the motte's elevation to create a commanding position, with the shell's design distributing weight evenly to prevent . While most shell keeps adopt a round shape for optimal stability and uniformity, variations occur to accommodate terrain, resulting in oval or irregular forms; for instance, Restormel Castle exemplifies the classic circular configuration. These structures were typically built using rubble stone bound with to ensure durability on the motte's summit.

Defensive and Residential Elements

Shell keeps incorporated defensive features that leveraged their enclosing stone walls to enhance protection while adapting to the constraints of mottes. A battlemented wall-walk, akin to a chemin de ronde, typically ran along the top of the walls, allowing archers and defenders to patrol and fire upon attackers from elevated positions. Occasional mural towers or postern gates provided additional security; for instance, at Trematon Castle, a postern gate on the west side offered a secondary, less conspicuous access point during sieges. These elements, combined with the circular or polygonal form of the walls, helped deflect siege engines and projectiles more effectively than earlier timber palisades. Within the enclosed space, shell keeps facilitated residential setups by utilizing the interior as a bailey-like for timber-framed buildings erected against the inner walls. These structures often included halls for communal gatherings, for religious observance, and private chambers for the and retainers; at Restormel Castle, the main hall featured large windows and a timber roof, while an adjacent included a and an east window, and upper-floor chambers served as a and wardrobe. This arrangement maximized space efficiency, with buildings reinforcing the walls structurally while providing living quarters. Unlike solid keeps, which prioritized isolated refuge, shell keeps served a multi-purpose role as both a last-stand defensive enclosure and the for Anglo-Norman lords, blending military, administrative, and domestic functions to symbolize feudal power and prestige. For siege endurance, adaptations included ground-level storerooms for provisions and water management features such as stone-lined pits and wells; Restormel's shell keep contained four such pits, one functioning as a well on the eastern side to ensure a reliable during prolonged attacks. These provisions allowed occupants to sustain themselves within the fortified shell, distinguishing it from more vulnerable open baileys.

Construction Methods

Materials Used

Shell keeps were primarily constructed using local stone rubble, such as or , which was readily available and cost-effective for building on artificial mottes. This was often sourced from nearby quarries or reused from earlier Roman structures, providing a practical supply of material while reducing transportation costs. The stones were bonded together with , produced by burning local or to create quicklime, which was then mixed with and to form a workable paste that allowed for flexibility in the structure. The walls of shell keeps typically featured an outer facing of —precisely cut and dressed stone blocks—for enhanced durability and a smooth appearance, while the inner core consisted of less refined to minimize overall weight on the motte and lower material expenses. This composite approach balanced structural integrity with the practical limitations of motte stability, as heavier solid stone constructions risked . Examples like Restormel Castle demonstrate this method, with its walls built from local slate-stone supplemented by dressed stone from regional sources. Timber elements, including and sweet , were essential for internal buildings, roofs, floors, and temporary during construction, sourced predominantly from nearby managed forests to ensure availability and quality. , prized for its strength and resistance to decay, formed the bulk of structural framing, while chestnut provided durable alternatives for less critical components. Regional variations in stone selection reflected local and resource accessibility, with commonly used in for its abundance, as seen in structures near and , and or preferred in the north, such as in the , for their resistance. Heavier stones like were generally avoided in shell keep designs to prevent overloading the motte, favoring lighter sedimentary options instead.

Building Techniques

The construction of shell keeps began with careful foundation preparation on the motte summit. Builders first leveled the uneven top of the artificial to create a stable platform, then excavated trenches around the perimeter for the wall bases, allowing the weight of the structure to be distributed more evenly across the underlying soft, compressible and reducing the risk of . This approach addressed the challenge of erecting heavy on an earthwork prone to shifting under load. The walls were erected layer by layer, with masons laying courses of stone using wooden scaffolding for access and support, hoisting materials via pulleys or simple cranes to reach higher levels. Unlike solid keeps, shell keeps employed a lighter design with thinner walls—typically 3 to 3.5 meters thick and 4.5 to 9 meters high—to avoid overloading the motte's unstable soil, while still providing enclosure for internal timber buildings. The core of these walls consisted of rubble packed with mortar, faced externally and internally with dressed ashlar blocks for durability and a smooth finish. Construction paused during winter months when mortar would not set properly, extending the overall effort. Labor for shell keeps drew on skilled masons, often itinerant specialists from Normandy or local regions, supplemented by unskilled local workers for quarrying and transport; royal or noble funding accelerated projects by providing resources and corvée labor. These upgrades to existing mottes were generally quicker than building full stone castles, taking about 1 to 2 years to complete the basic shell structure, though additional internal features could extend the timeline. Key innovations included the use of temporary wooden —falsework frameworks—to support the voussoirs during the of arched entrances and gateways until the mortar cured, enabling spans that enhanced defensibility without compromising the lightweight design. Post-construction, any incidental voids in the wall cores were filled with compacted earth or additional to bolster stability against settlement.

Distribution and Examples

Sites in England

Shell keeps in England represent a significant architectural adaptation during the 12th century, with around 70 examples identified across the , the majority in , predominantly constructed amid the instability of (1135–1153), when rapid fortification was essential for defense. These structures were often erected on existing mottes of motte-and-bailey castles, transforming wooden palisades into stone enclosures to enhance security without the need for massive square keeps. Their distribution reflects Norman influence, particularly from designs inspired by Falaise Castle in , emphasizing circular or polygonal walls to evenly distribute weight and maximize defensive perimeters. One of the best-preserved examples is Restormel Castle in , featuring a near-complete circular shell keep with 8-meter-high walls dating to the early , enclosing a central originally housing timber buildings. in also preserves a circular shell keep constructed around 1153 by Roger de Berkeley during , utilizing the motte's natural elevation for strategic oversight of the . The keep's surviving walls, up to 10 meters high in places, enclose an inner that originally housed timber buildings for residence and storage, demonstrating the transitional phase from wooden to stone fortifications. Its well-maintained state allows modern visitors to appreciate the original Norman layout, with later 14th-century additions integrating seamlessly. At in , a 12th-century shell keep forms the foundation of the iconic , initially built under Henry II (r. 1154–1189) to replace earlier wooden defenses on the motte. This structure, later rebuilt and heightened by Edward III in the 1350s, exemplifies how shell keeps evolved into more elaborate towers while retaining their core function as a high-level stronghold overlooking the Thames. The integration of the shell keep with subsequent concentric defenses highlights its role in expanding royal authority during a period of consolidation post-Anarchy. Lincoln Castle features a unique dual-motte configuration, with the southwestern motte topped by the Lucy Tower, a polygonal erected in the mid-12th century to serve as the bishop's residence, separate from the royal square keep on the eastern motte. This arrangement, constructed around 1150 amid the Anarchy's conflicts, allowed segregation of royal and episcopal forces, preventing internal disputes within the fortress while providing mutual defense against external threats. The Lucy Tower's remnants, including curved walls and a foundation, underscore its dual and purpose. Other notable sites include in , where William d'Aubigny II built a circular shell keep around 1140 on a high motte, influenced by Norman prototypes and featuring buttresses for added stability. Castle in also preserves a shell keep from the mid-, enclosing the motte with a curtain that supported internal timber halls, reflecting the de Warenne family's efforts to fortify during turbulent times. in features a shell keep with internal ranges around a , dating to the and illustrating residential adaptations. In contrast, some shell keeps have been lost to demolition, such as at , where the 12th-century polygonal enclosure was razed in the early to create landscaped grounds, leaving only the motte visible today. Lewes Castle in East Sussex shares Lincoln's dual-motte design, with shell keeps on both mounds developed by the de Warenne family around 1100–1150 to separate baronial and possibly administrative functions, strategically positioning the castle to control the Ouse Valley and deter invasions. This layout, rare in England, emphasized compartmentalized defense, allowing independent operation of each motte while sharing a common bailey for coordinated resistance. Overall, these English sites illustrate the shell keep's practicality in an era of civil strife, with many originating as hasty upgrades to pre-existing earthworks.

Sites in Wales and Beyond

Shell keeps in were primarily constructed by Norman lords during the 12th and 13th centuries to assert control over the volatile marcher lands along the English-Welsh border, adapting the motte-and-bailey design to rugged terrain and frequent Welsh resistance. Notable examples include , where a distinctive twelve-sided shell keep was erected around 1090–1111 by , enclosing internal buildings within its 3.7-meter-thick walls and providing panoramic views from its motte. Tretower Castle in features a mid-12th-century shell keep built by the Picard family on an earlier motte, later supplemented by a and towers as the site evolved into a fortified residence. Castle in retains footings of a 13th-century shell keep enclosure, refortified after Welsh attacks to secure the strategic Tywi Valley crossing. Wiston Castle, attributed to the Flemish settler in the late , preserves a well-maintained stone shell keep up to 4 meters high on its motte, exemplifying frontier adaptations with minimal internal subdivisions. Beyond Wales, shell keeps are rare, reflecting their concentration in Britain due to the prevalence of mottes in post-Conquest landscapes, though Norman influences extended to limited sites in Scotland and the European continent. Shell keeps are concentrated in England and Wales, with fewer examples in Scotland and continental Europe. In Scotland, Duffus Castle in Moray boasts a unique square 14th-century stone shell keep on a massive motte, constructed by the Moray family as a defensive stronghold amid regional conflicts. Continental examples are scarcer, largely confined to Normandy's flatter terrains where mottes were less common, but include Château de Pirou's well-preserved shell keep from the 12th century, featuring a circular donjon annulaire enclosing a courtyard on an artificial island. Similarly, Château d'Harcourt in Normandy retains an octagonal shell keep dating to the late 12th century, one of Europe's best-preserved examples, built by the Harcourt family to guard Norman borders. Preservation efforts in , particularly through 20th-century excavations and restorations, have illuminated the internal layouts of these shell keeps, often revealing timber-fringed buildings and gates within the enclosing walls. At , early 20th-century restorations by the Bute family exposed the keep's phased construction and domestic features. Tretower's shell keep benefited from -led archaeological work in the mid-20th century, uncovering evidence of its transition from military to residential use. These interventions, alongside surveys at and Wiston, have highlighted how shell keeps in frontier prioritized rapid fortification over elaborate design, with many sites now managed by for public access and further study.

Significance and Modern Study

Military and Strategic Importance

Shell keeps served as elevated command posts atop mottes, offering strategic oversight of surrounding frontiers and territories due to their commanding height, which facilitated and rapid response to threats. This positioning allowed lords to monitor borders effectively, particularly in contested regions like the , where they functioned as part of a broader "defense-in-depth" , providing refuges for local populations and bases for counterattacks against incursions. Typical garrisons in these structures were small, sufficient for maintaining control over key routes while leveraging the motte's natural defensibility. The enclosure design of shell keeps provided military advantages through flexible defensive arrangements, with circular stone walls enabling defenders to man wall-walks and battlements for enfilading fire, outperforming isolated square towers in resisting assaults by distributing defensive efforts across a perimeter. This adaptability proved valuable in prolonged conflicts, such as (1135–1153), where shell keeps like that at Carisbrooke withstood sieges and supported royalist or baronial forces amid civil unrest. Their fire-resistant construction also mitigated vulnerabilities of earlier wooden fortifications, allowing effective housing of troops and supplies during frontier skirmishes. Beyond tactical utility, shell keeps embodied Norman authority, their imposing silhouettes visible from afar serving as symbols of and deterrence against rebellions by projecting unassailable lordship over subjugated lands. However, these structures had limitations, including vulnerability to beneath the motte during , which could destabilize the foundation, and their relatively modest scale restricted prolonged resistance without external aid. By the 13th century, advancing and preferences for more robust rectangular donjons rendered shell keeps obsolete, leading to their replacement or abandonment in favor of concentric designs.

Preservation and Archaeological Insights

In the 20th and 21st centuries, archaeological excavations have provided critical insights into the internal structures of shell keeps, often revealing evidence of timber buildings and domestic arrangements that were otherwise obscured by later modifications or demolitions. At , rescue excavations conducted between 1989 and 1990 uncovered limited traces of timber foundation features within the shell keep, highlighting the challenges of interpreting post-medieval overbuilding on these sites. Similarly, investigations at Restormel Castle have emphasized its exceptionally well-preserved late 13th-century radial plan, including standing remains of a hall, kitchen, chapel, and chambers built against the inner wall, which demonstrate a high-status residential layout rather than a mere defensive shell. These digs, alongside surveys at sites like (1954) and Launceston (2006), have used geophysical and fabric analysis to map internal footprints, filling gaps in historical records where is sparse. Preservation of shell keeps faces significant challenges, including natural of underlying mottes and the impacts of modern , which can accelerate wear on and earthworks. Mottes, often artificial mounds susceptible to slippage and , require ongoing stabilization to prevent collapse, as seen in broader motte castle studies where instability has been documented through modern surveys. Many shell keeps, such as those at Carisbrooke and Halton, are protected as Scheduled Monuments under the Ancient Monuments and Archaeological Areas Act 1979, administered by , which prohibits unauthorized works and mandates conservation plans to mitigate these threats. , while boosting awareness, contributes to footpath and requires managed access to balance public engagement with site integrity. Modern scholarship, notably Robert Higham's 2016 analysis, has refined our understanding by estimating around 20 surviving shell keep sites, primarily in , while acknowledging debates over exact counts due to post-medieval demolitions and reinterpretations of ambiguous mottes. Higham's work critiques earlier assumptions, proposing that shell keeps functioned more like miniaturized baileys with central open spaces for activities, based on integrated architectural and excavation data. These studies have also addressed historical misconceptions by revealing evidence of sophisticated domestic features—such as fireplaces, garderobes, and arrow-loops—that contradict views of shell keeps as temporary or transitional structures, instead portraying them as enduring elite residences integral to daily medieval life.

References

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