The keep of the Bishops’ residence Ziesar Castle is visible from afar. The 35-metre tall fieldstone tower was built around the year 12 and stands in the south-east of the castle complex. Its so-called Renaissance-style bishop's cap is striking, a former watch chamber at the top of the tower. Whereas the castle was formerly used as a refuge for the inhabitants of the castle, in the 19th century it was even used as a chimney for the neighbouring starch factory, before it was restored in 2000 and made accessible to visitors again. The keep can be climbed as an observation tower when the museum is open and provides a very beautiful views over the town of Ziesar and the Fiener Bruch lowlands.