Castra of Buch, Roman military fort in Rainau, Germany

The Castra of Buch is a Roman military fort near Rainau situated at around 462 meters elevation. The installation was part of the defensive network of Raetia with structured spaces for administration and quarters.

The fort was built as part of Roman border defense in Raetia and played an important role in monitoring the region. Its layout followed the standard Roman military architecture of this frontier zone.

The fort's civil settlement, the vicus, lay in the valley below the promontory, near where the Ahlbach meets the Jagst. Its bath building, about 100 m north-east of the fort's north-east corner, was also used by civilians and went through four construction phases. Foundations of the baths and two neighbouring vicus buildings were preserved after the 1979–1980 excavations and can be seen freely.

The excavated south gate, a stretch of the defensive wall and the foundations of the baths and two vicus buildings were conserved after the 1972 and 1979–1980 digs and can be viewed freely. The fort lies on a spur above the Jagst valley, where the Ahlbach meets the river; today the Rainau-Buch reservoir fills the valley floor. The Limes-Park Rainau links the site with the Dalkingen Limes Gate about 2.25 km to the northeast.

Its location along the boundary between Roman and non-Roman territory made this site a strategic observation point. Archaeologists have uncovered traces of daily life here showing how soldiers lived at this remote station.