A keep is a type of fortified tower built within castles during the Middle Ages by European nobility. Scholars have debated the scope of the word keep, but usually consider it to refer to large towers in castles that were fortified residences, used as a refuge of last resort should the rest of the castle fall to an adversary. The first keeps were made of timber and formed a key part of the motte-and-bailey castles that emerged in Normandy and Anjou during the 10th century; the design spread to England, Portugal, south Italy and Sicily. As a result of the Norman Conquest of England in 1066, use spread into Wales during the second half of the 11th century and into Ireland in the 1170s. The Anglo-Normans and French rulers began to build stone keeps during the 10th and 11th centuries, including Norman keeps, with a square or rectangular design, and circular shell keeps. Stone keeps carried considerable political as well as military importance and could take a decade or more to build.

During the 12th century, new designs began to be introduced – in France, quatrefoil-shaped keeps were introduced, while in England polygonal towers were built. By the end of the century, French and English keep designs began to diverge: Philip II of France built a sequence of circular keeps as part of his bid to stamp his royal authority on his new territories, while in England castles were built without keeps. In Spain, keeps were increasingly incorporated into both Christian and Islamic castles, although in Germany tall fighting towers called bergfriede were preferred to keeps in the western fashion. In the second half of the 14th century, there was a resurgence in the building of keeps. In France, the keep at Vincennes near Paris began a fashion for tall, heavily machicolated designs, a trend adopted in Spain most prominently through the Valladolid school of Spanish castle design. Meanwhile, tower keeps in England became popular amongst the most wealthy nobles: these large keeps, each uniquely designed, formed part of the grandest castles built during the period.

In the 15th century, the protective function of keeps was compromised by improved artillery. For example, in 1464 during the Wars of the Roses, the keep of Bamburgh Castle on the Northumberland coast, previously considered to be impregnable, was defeated with bombards. By the 16th century, keeps were slowly falling out of fashion as fortifications and residences. Many were destroyed in civil wars between the 17th and 18th centuries or incorporated into gardens as an alternative to follies. During the 19th century, keeps became fashionable once again, and in England and France, a number were restored or redesigned by Gothic architects. Despite further damage to many French and Spanish keeps during the wars of the 20th century, keeps now form an important part of the tourist and heritage industry in Europe.

Since the 16th century, the English word keep has commonly referred to large towers in castles. The word originates from around 1375 to 1376, coming from the Middle English term kype, meaning basket or cask, and was a term applied to the shell keep at Guînes, said to resemble a barrel. The term came to be used for other shell keeps by the 15th century. By the 17th century, the word keep lost its original reference to baskets or casks and was popularly assumed to have come from the Middle English word keep, meaning to hold or to protect.

Early on, the use of the word keep became associated with the idea of a tower in a castle that would serve both as a fortified, high-status private residence and a refuge of last resort. The issue was complicated by the building of fortified Renaissance towers in Italy called tenazza that were used as defences of last resort and were also named after the Italian for to hold or to keep. By the 19th century, Victorian historians incorrectly concluded that the etymology of the words "keep" and tenazza were linked and that all keeps had fulfilled this military function.

As a result of this evolution in meaning, the use of the term keep in historical analysis today can be problematic. Contemporary medieval writers used various terms for the buildings we would today call keeps. In Latin, they are variously described as turris, turris castri or magna turris – a tower, a castle tower, or a great tower. The 12th-century French came to term them a donjon, from the Latin dominarium "lordship", linking the keep and feudal authority. Similarly, medieval Spanish writers called the buildings torre del homenaje, or "tower of homage". In England, donjon turned into dungeon, which initially referred to a keep, rather than to a place of imprisonment.

While the term remains in common academic use, some academics prefer to use the term donjon, and most modern historians warn against using the term "keep" simplistically. The fortifications that we would today call keeps did not necessarily form part of a unified medieval style, nor were they all used in a similar fashion during the period.

The earliest keeps were built as part of motte-and-bailey castles from the 10th century onwards – a combination of documentary and archaeological evidence places the first such castle, built at Vincy, in 979. These castles were initially built by the more powerful lords of Anjou in the late 10th and 11th centuries, in particular Fulk III and his son, Geoffrey II, who built a great number of them between 987 and 1060. William the Conqueror then introduced this form of castle into England when he invaded in 1066, and the design spread through south Wales as the Normans expanded up the valleys during the subsequent decades.