Castra Arbon, Roman fortress ruins in Arbon, Switzerland

Castra Arbon is the late Roman fort of Arbor Felix, built into the Danube-Iller-Rhine Limes on a moraine ridge above Lake Constance. Nothing of it stands free: the medieval town centre covers the whole area, with Arbon Castle over the northern part and the Martinskirche and Gallus chapel over the southern part, a former cemetery between them. Its existence was only proven archaeologically in 1957.

The fortress was built around 280 AD to protect the Roman Empire's northern border against invaders. It was part of a large defense system stretching along the Danube, Iller, and Rhine rivers that stood for several centuries.

The Latin name Arbor Felix, "blessing-bringing tree", appears in the Itinerarium Antonini of the 3rd century and may derive from the Celtic place name Arbona. Roman priests buried their cut hair and nails under such a tree. The Notitia dignitatum lists a tribune commanding a cohort of Pannonians at "Arbor Felix".

The castrum has no open excavation to visit: it lies entirely beneath Arbon's medieval centre. The north part is covered by the town castle, the south by the Martinskirche and Galluskapelle, with the castle moat and castle park in between. Look for these landmarks to trace the Roman footprint.

The castrum was only archaeologically proven in 1957, making it one of the youngest and most important discoveries of late Roman military installations in Switzerland. Its name Arbor Felix, first recorded in the 3rd-century Itinerarium Antonini, may derive from the Celtic place name Arbona.