Schloss Arbon is a sturdy medieval tower house on a low promontory jutting into the lake, its pale limestone walls visible from the water and from the lakeside promenade below. The building dates largely from the sixteenth century, but it stands on Roman foundations — and in the basement you can see the remains of a Roman watchtower that has been standing since the third century.
The castle houses the town's history museum, with collections covering the Roman period, early medieval archaeology, and the industrial heritage of the Saurer works that once employed most of Arbon. The exhibits are modest but well-presented. Entry is inexpensive and the views from the tower over the lake are worth the climb alone.