Mouse Tower, Gothic brick tower near Lake Gopło, Kruszwica, Poland

Mouse Tower is a 32-metre octagonal brick tower on the Rzępowski Peninsula of Lake Gopło in Kruszwica, the only remnant of the town's castle. Its exterior is octagonal while the interior is round, and the small holes spaced across the reddish-brown brickwork are traces of scaffolding, not windows. A dark metal spiral staircase climbs the left side to an opening partway up, and the base rests on rough gray stone.

A first stone tower on a circular plan was built in the mid-13th century. The present brick tower was erected around 1350 by King Kazimierz the Great as part of a castle guarding Kruszwica against the Teutonic Knights; after the order's decline the castle became the seat of the Kruszwica castellany and starosty. Occupied during the first two years of the Swedish Deluge, the castle was blown up in 1657, and the tower survived as its only element.

The tower takes its name from a Polish tale about Prince Popiel II, who according to old stories was eaten by mice as punishment for his cruelty. This legend has turned the structure into a symbol of folklore and moral stories that connects visitors to early Polish narrative traditions.

Visitors reach the site via marked paths from central Kruszwica, and the climb to the top involves a narrow spiral staircase inside the structure. Guided tours run between April and October and provide explanations about the construction and the view over the lake and surrounding countryside.

The holes dotting the brickwork are not windows but the sockets left by the wooden scaffolding used during construction. The tower is octagonal outside yet round inside, and since 1895 it has served as a viewing point from which Inowrocław, Strzelno and Radziejów can be seen on clear days.