This castle, built in the 8C by Radboud (King of the Friesians), stands on the opposite side to the Oosterhaven. It was fortified around 1288 by the Count of Holland Floris V, who transformed it to hold the inhabitants of western Friesland, who had just been conquered. Only a part of the building remains today, surrounded by moats. The rest was destroyed in the 17 and 18C The castle today houses a museum made up of coins and other objects from excavations there.