Sparrenburg Castle is the most famous landmark. It was built between 1240 and 1250 by Count Ludwig von Ravensberg. The 37 m high tower and the catacombs of the castle are open to the public. The oldest city church is Altstädter Nicolaikirche. It is a Gothic hall church with a height of 81.5 m. It was founded in 1236 by the Bishop of Paderborn, and enlarged at the beginning of the 14th century. The church was damaged in World War II and later rebuilt. Three times a day, a carillon can be heard. The most valuable treasure of this church is a carved altar from Antwerp, decorated with 250 figures. A small museum housed within illustrates the history of the church up to World War II.