Close to Korsør there are a number of historic remains from times of unrest. On the isle of "Sprogø" and near "Tårnborg" walls and patterns in the ground confirm the existence of two castles from the twelfth century meant to control the passage through the Great Belt. – Close to the town centre are the remains of Korsør Castle from the thirteenth century with its impressive old tower still intact and two miles north of Korsør entrenchments from the war with Sweden in 1658 are still visible.