Some archaeological research proved that a settlement in the area of the Castle Hill existed as early as in the period of the Lusatian culture (Iron Age 650-400 BC).
So called Bavarian Geographer mentioned the tribal name Glopeani and 400 early-towns around Lake Goplo, including the town of Kruszwica in the middle of the 9th century.
The beginnings of the fortified town on the Castle Island in Kruszwica date back to the early Middle Ages. The very first note about the town is from 1096, when the dynasty fights between Wladyslaw Herman, the ruler of the Piast State with his son Zbigniew resulted in the destruction of Kruszwica. The second note is from 1271, when Boleslaw the Pious ordered the fire of Kruszwica.
Casimir the Great’s Gothic castle was built in the second half of the 14th century on a part of the former town hill. It was designed on an irregular projection of a polygon with a surface area of about 0.37ha. The castle hill was raised for about 4m. A new canal – a moat was dug up in order to separate the hill from the area outside town walls. The remains of the former round tower were enriched with huge brick defensive walls (11m high, topped by battlements) on the base course made of erratic blocks. In the northern protruding part of the walls, there was a gate with a building flanked by a gate tower with a buttress. Later, a porch and a drawbridge were added. Then, an octagonal brick walls were built around the stone Mouse Tower and a steep ogive roof with a hoard was added. The castle buildings were built on as adjacent to the defensive walls from the inside: the southern (the main) one had a double roof flanked by turrets in the corners, including a turret on the diagonal buttress. The eastern building was adjacent to the gate building. There was a castle kitchen adjacent to the eastern wall of the main building and a detached bath house in the castle courtyard. Some outbuildings of either a framework or wattle and daub construction were added along the defensive walls. The northern brick wing of the castle was also added to the main building in the Middle Ages. The castle was on fire in 1519 and 1591. It was rebuilt after each conflagration. The final destruction of the castle was brought about by the Swedes, who captured Kruszwica in 1655 and in 1657 they set the castle on fire. The oldest image of the castle dates back to 1655 and it is recorded in a print made by Willem Swidde in 1694.
In 1772, at the First Partition of Poland, Kruszwica was incorporated into the Kingdom of Prussia. The remaining parts of the castle walls and the Mouse Tower started to be taken down in 1787 to be used for building houses in the area, including a manor house in Zagople for a German owner of an estate in Kruszwica. Between 1791 and 1796, the Prussian authorities issued a directive on protection of the castle ruins. The Mouse Tower was underpinned in 1802. Some parts of the castle walls were uncovered in 1867, then the castle hill was levelled and converted into a park. Now, the Mouse Tower with two parts of the defensive walls is well-preserved. In the western wall, some tracks of window alcoves in the northern wing of the main castle building survived and in the northern wall, a track of a ridge in the roof of an outbuilding also preserved. Some other parts of the castle were restored and made available for visiting in 2011. These are the interiors of the basement of the western part of the main building, some parts of the gate construction and the porch as well as the reconstruction of the wooden bridge and the stone base courses of the defensive walls.

The most important historical facts:
about  976 – establishment of the Early Piast dynasty times town
1096 – destruction of Kruszwica during the fight between Wladyslaw Herman with his son Zbigniew
1148 – wedding of Boleslaw Wrymouth’s daughter with Otto, Albrecht the Bear’s son
1271 – fire of Kruszwica on the orders of Boleslaw the Pious
1332 – destruction of the town by the Teutonic Order
1368 – the privilege for the Cistercian convent granted by Casimir III the Great
1371 – the fire of the castle
1422  - the Magdeburg rights granted by Wladyslaw Jagiello
1519 – the fire of the castle and the reconstruction of the castle
1591 – the fire of the castle and the reconstruction (new building)
1616 – examining of the castle, ‘(...) building of the new, 3 storey high next to the tenement; building of the old, a bath house and a tower for 8 edges (...)’
1655 – Swedes’ occupation of the castle
1657 – the fire of the castle caused by the Swedes on their leaving
1772 – the 1st Partition of Poland, incorporation of Kruszwica into the Kingdom of Prussia
1791/96 – the Prussian directive on protection of the castle ruins, a clean of the Castle Hill
1896 – restoration of the Mouse Tower and making it available for visiting
1956 – building of new spiral reinforced concrete stairs leading to the tower
2003-2010 – archaeological and conservation works on the Mouse Tower and its surroundings
2011 – revitalization of the part of the basements and the castle gate with the reconstructed drawbridge and the part of the defensive walls