The Mouse Tower in Kruszwica: The most famous historical building in Kruszwica is the 32-meter high tower known as the “Mouse Tower” and small ruins of the castle. that are the remains of the object constructed during the reign of Kazimierz III Wielki. This was one of the most powerful fortresses protecting the Kingdom of Poland from the Teutonic Order and was located on the then Rzępowska Island on the Gopło Lake. After the Second Peace of Thorn its meaning diminished. however it fell into ruin only after the Swedish “deluge”. and its bricks were used as building material; in 1802 the tower was strengthened to prevent its collapse. In the 18th or 19th century the legend of Popiel was attributed to the tower and it was considered the place of death of this legendary ruler.