The Castra Alteium (German: Kastell Alzey) is a former late-Roman border fort on the Danube-Iller-Rhine Limes (DIRL). It is located in the territory of the city of Alzey in Rhenish Hesse, Germany. The fort was presumably built in the course of the last reconstruction measures on the Rhine frontier between 367 and 370 AD under the western Emperor Valentinian I. Previously, there was a Roman civilian settlement (Vicus), Altiaia, which was devastated by Alamanni in 352–353. The fort was also destroyed twice, and probably abandoned at the end of the fifth century.

The ancient place name Altiaia possibly goes back to a pre-Roman Celtic settlement from 400 BC. The Roman name appears for the first time on the dedication inscription of a Nymphaeum reused in the fort wall. The inscription, identifying the population as vicani Altiaienses, and the town as vicus Altiaiensium or vicus Altiaiensis is datable to the year 223. The meaning of the name can no longer be determined today. The late antique Alteium (or Altinum ) is mentioned only in Codex Theodosianus and is almost certainly derived from the name of the civil settlement. In the Codex the place is once referred to as Alteio and the other time again as Altino.

The town of Alzey is located in German federal state of Rhineland-Palatinate on the western edge of the northern part of the Upper-Rhine Valley (Oberrheinischen Tiefebene) on the left side of the Rhine, about 30 kilometres (19 mi) from it. It is surrounded by the northern part of the Alzey hill country, which is adjoined to the north by the Rheinhessisches Hügelland and to the west by the Nordpfälzer Bergland. The town is located about 30 kilometres (19 mi) southwest of Mainz and about 22 kilometres (14 mi) northwest of Worms. Through Alzey flows, partly underground, a section of the Selz, a left tributary of the Rhine. The narrow Selz valley begins to widen from Alzey to the north. The Roman civilian settlement was part of the province of Germania Superior and was governed from the provincial capital of Mogontiacum (Mainz). After the administrative reforms of Diocletian Castra Alteium was located within the territory of the new province of Germania Prima in the southwestern area of the former vicus, on a southern spur of the Mehlberg mountain on a steep slope to Selz. From here the garrison had a good view of the surrounding area, especially to the north.

The fort probably protected and monitored a crossing over the Selz and the junction of the road links Mainz-Alzey-Metz and Bingen-Kreuznach-Alzey-Worms. However, the camp was possibly used primarily for the temporary accommodation of units of the mobile field army (Comitatenses), because there was otherwise little accommodation in the hinterland of Mogontiacum for larger troop contingents. In an emergency, the central plaza could also accommodate tents to quarter additional troops.

Datable finds of the vicus date back to the middle of the 4th century AD. The first known reports on Roman finds were written in 1783 by the pastor of Dautenheim, Johann Philipp Walther, who excavated old foundations on a church-owned field (presumably the remains of the eastern wall of the fort) and discovered three Roman inscriptions. In 1869, the Mainzer Altertumsmuseum acquired late-antique or early medieval finds from Alzey— a pair of gilded-silver crossbow brooches, a silver needle, earrings, two small disc brooches with almandine inlay, and pendants, each richly decorated with golden filigree wire. Such crossbow brooches were worn only in pairs at the shoulders. Together with two small brooches, such as small disc brooches, they formed the "vierfibeltracht" which was typical of the women's fashion in the 6th century. In the 1870s the teacher Gustav Schwabe presented a collection of Roman finds, which were later lost. In 1871/1872 a votive altar to the goddess Sulis came to light in the north wall. Another altar found in Alzey was dedicated to Fortuna. Another example was dedicated to Minerva by the fuller Vitalianus Secundinus. In the foundations of the late antique church building in the fort they encountered fragments (spolia) of a gateway, which was probably originally erected in a place of worship of the god of the springs Apollo-Grannus, probably the sulfur spring at today's tax office.

In 1902 the section commissar of the Reichs-Limeskommission (RLK), Karl Schuhmacher (1860-1934), and the local historian Jakob Curschmann (1874-1953), identified a part of the wall and the foundations of a round tower at the southwest corner. Nursery owner Jean Braun, then the owner of the fort and later co-founder of the Alzey Museum, continued to investigate and discovered further remains of the wall on the west side. By 1904, during construction, further remains of the fort wall and, at the cemetery of the former St. George's Church, ancient sandstone slabs and sarcophagus components came to light. In 1904, the preservationist Soltan dug large parts of the eastern wall. The east gate was only very poorly preserved; it could only be stated that the gate towers extended behind and in front of the wall. In 1906, further foundations of the fort were discovered and partially restored.

In 1909, the prehistorian Eduard Anthes (1859-1922) took over the supervision of the excavations, supported by the district and city of Alzey, the Historical Association of the Grand Duchy of Hesse, and the Römisch-Germanischen Kommission excavations. In the same year Braun also discovered the west gate, whose passage was mostly filled with rubble. The southeastern corner tower was of high structural quality and its existing masonry still exhibited several courses. On the south wall, Braun discovered two well-preserved rooms of a barracks attached to the castle wall. The two rooms were dug to a depth of 11.5 metres (38 ft). At the bottom of the eastern chamber many animal bones were found; presumably this part of the building served as a slaughterhouse. In the western iron fragments and tools as well as two stones which may have served as anvils were uncovered. In front of the building was a well shaft covered with sandstone slabs. By 1909, about 62 metres (203 ft) of the perimeter wall had been uncovered. At most of the sites examined it was only 20–30 centimetres (7.9–11.8 in) below the surface. Their rising masonry was still partially preserved up to a height of 50–60 centimetres (20–24 in). In 1925, the prehistorian Wilhelm Unverzagt (1892-1971) succeeded in finding the so-called "Alzey burn layer" which marked the end of the second settlement phase of the fort. Above this layer mainly ceramics of the late 4th century AD were recovered. The complex of migration period ceramics from the excavations is still used in research today as a tool to date other sites of this era. Several excavation campaigns in the fort area were also conducted by the Institut für Vor- und Frühgeschichte of the Johannes Gutenberg University of Mainz.

For the dating of the fort mainly the coin finds and a brick temple of the Legio XXII Primigenia were important. The objects found in the excavation area, mainly Roman glassware and ceramics, give some information about the origin of the fort's inhabitants. Noteworthy in this context is a comb with a bell-shaped handle, which was widespread among the East Germanic peoples. Other comb types from Alzey come from Elbe Germanic regions. Half-round, serrated belt buckle plates of the "Muthmannsdorf" type have been mainly observed on the Danube and in the Elbe and eastern Germanic areas, but provincial Roman types are also represented. In 1929 a 5 x 11.5 cm limestone slab with three engraved busts and two Christ monograms was discovered in the southeast corner of the fort. Also found was an early Christian bread stamp from the 4th century used to stamp the eucharistic bread.. The discovery of Spiral fibulae of the "Mildenberg" type, which did not originate before 440 AD, marks the Alamannic settlement phase of the fort.