Irgenhausen Castrum is a Roman fort situated on Pfäffikersee lake shore. It was a square fort, measuring 60 metres in square, with four corner towers and three additional towers. The remains of a stone wall in the interior were probably a spa.
In the Roman era, there was a Roman road from Centum Prata (Kempraten) on Obersee–Lake Zürich via Vitudurum (Oberwinterthur) to Tasgetium (Eschenz) on the Rhine. To secure this important transport route, the castrum was built. The native name of the fort is unknown: Irgenhausen was mentioned in AD 811 as Camputuna sive Irincheshusa, so maybe the castrum's name was Cambodunum, the Roman name of the neighboring village of Kempten.
For the dating of the fort there are two theories: the first assumes that the fort was built at the time of the Emperor Diocletian around AD 294/295. The other theory, based on the Roman coins found inside the castrum, dated the construction from 364 to 375, in the era of the Emperor Valentinian II. As early as AD 400 the castrum was evacuated and destroyed by Alamanni invaders.
In addition to the remains of the towers and surrounding wall, there were found the remains of stone interior buildings: a three-roomed building was seen as a spa. Another building with three rooms has been interpreted as principia, the headquarters of the fort. At the southern corner tower a hypocaust system of an older villa rustica from the 1st to the 3rd century was excavated. The other buildings were made of wood and therefore cannot be individually identified. However, some military barracks, a horreum and a praetorium was probably built inside the fort. In the middle of the hill there was a sunken room. Most of the relics found inside the fort date from the 2nd and 3rd centuries AD and are thought to be relics of the villa rustica on whose ruins the fort was built. At the present time, a red ribbon in the wall shows where the Roman wall ends and the restored wall begins.
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