The earliest tracks of the settlement in the area of Kruszwica date back to the period called the final Paleolithic (11,000 BC to 8,000 BC). They were left by the first hunters, so called reindeer hunters!
Fertile soil and natural resources (underground salt water) as well as the lake rich in fish and the crossing of some routes (from Wielkopolska, or Greater Poland to Rus and the Amber Road, which was mentioned by Etruscans in the 2nd century BC) created favourable conditions for settlement in this area. Lake Goplo made the travelling easier as it was a part of the waterway from the Warta river to the Vistula river. In the 10th and 11th centuries, the town was a residence of the Polish kings and dukes.
The town was built in late 970s as sedes regnii principales, that is, one of the few princely residences belonging to the early Piast dynasty. It was an important defensive, economic, administrative and church centre at the early Piast dynasty times. The town was destroyed during the dynastic fights at the end of the 11th century, however, because of the importance of this part of Kuyavia, it still existed as a non-fortified but thriving early-town. There were a lot of various handicraft workshops and glass making workshops (perhaps the very first in Poland); also, the trade flourished here (especially with the Rus).
In the 11th century, St. Vitus church, the very first stone church was built inside the town walls. Some other residential buildings may also have been built at that time.
The later structure, mentioned among others, by Jan from Czarnkow, a chronicler, was the Mouse Tower, built inside the town walls perhaps in the 13th century (before the time Kruszwica was overtaken by Wladyslaw Lokietek); it was made of stone and it was round at the bottom.