Castra Ad Fines, Roman military fort in Pfyn, Switzerland.

Castra Ad Fines is a late Roman fort on the eastern part of a drumlin called Bergli, about 20 metres above the Thur. Its northern rampart survives in places, but the interior is almost entirely covered by the medieval village centre and the Sankt Bartholomäuskirche.

The fort was established to control the border between two Roman provinces and operated as a crucial outpost for centuries. It was abandoned after 400 CE as power structures shifted and Roman control in the region weakened.

The name Pfyn derives from the Latin ad fines, "at the border", since the line between the provinces of Raetia and Belgica ran nearby. The site appears as a staging post on the Augsburg–Pfyn–Trier route in the Itinerarium Antonini and the Tabula Peutingeriana, where distances switch from Roman miles to Gallic leagues.

The fort lies beneath the medieval village centre and the Sankt Bartholomäuskirche, about 20 metres above the Thur; only remnants of the northern rampart are visible. The Thur now runs some 500 metres south of the Städtlihügel because of 19th-century river regulation. Pfyn is about six kilometres from Frauenfeld.

The fort's interior was densely built up and appears not to have been used solely by the military. Finds there were mainly coins, tableware, jewellery, toys and tools, with only a few weapons, suggesting families and craftspeople lived alongside the garrison.