Irgenhausen Castrum

Late Roman square fort (c. 61 m per side) overlooking Pfäffikersee in the canton of Zürich, built in the 4th century over the ruins of a 1st-century villa rustica destroyed during the Alamanni raids of AD 259–260. The castrum had four corner towers (8 m x 8 m), mid-wall towers, and a 1.9 m thick enclosing wall, guarding the military road from Kempraten (Centum Prata) via Rapperswil to Vitudurum (Oberwinterthur) as part of the Danube-Iller-Rhine Limes. Interior structures included a possible bath complex with three apsed rooms and wooden barracks. Abandoned c. AD 401 when Roman troops withdrew to Italy, it is the best-preserved late Roman fort in northeastern Switzerland. Excavated 1898–1908 and placed under federal protection in 1909.