Frederick II built this castle on the rocky promontory next to the cathedral in 1233, partly as a fortified residence and partly to assert Hohenstaufen power over the trading port. The squat square keep with four corner towers is a textbook Swabian castle: clean lines, thick walls, almost no decoration. The interior holds a small archaeological collection — sculptures, medieval coins, and a stone fragment said to be from the original castle prison where Manfred of Sicily was held. The terrace looks straight back over the harbour to the cathedral. A modest entry fee.