In 610, Irish monks who were followers of Columban of Luxeuil settled in Arbon. They found a small Christian settlement called "Castrum" there. One of these monks was St. Gallus, the founder of the monastery of St. Gallen. He died in 627 in Arbon. In 720, a fortification on the site of the present castle was built by a Frankish troop. The foundations of this early fortress remain.