The museum is headquartered in a towerless castle built on the south-eastern shore of the Gosławickie Lake between 1420-1426 by one of the most prominent diplomats of the Jagiellonian era, the Bishop of Poznań, Andrzej Łaskarz, of the Godzięba coat of arms. The construction was erected in sandy shallows, protected by swamps in the west and the lake in the north. From the south, the access was obstructed by the swamps and, probably, a moat. There were no natural obstacles east of the castle.

The castle consists of two buildings situated one opposite the other in the north-south direction and linked through a southern extension. All the mentioned structures had basements (the basement of the extension used to be a dungeon). North of the castle, there is a small inner courtyard with a well. Originally, the area was protected by an entrance gate. The courtyard rises about 2.5 m (8.2 ft.) above the ground level between the outer defensive walls and inner walls of the castle.The defensive wall surrounding the castle was probably erected during the second stage of the castle construction work, after the bishop's death. On its corners there were cylindrical turrets supported by consoles. The walls, apart from the eastern part, are all of a similar construction: a smooth outer face, arcades strengthening the walls and supporting the parapet walk above. The southern part, is thicker than the western and the northern (by 1.4m [4.6 ft.] and 0.95 m [3.1 ft.] respectively). This part of the fortification also has loopholes. The eastern defensive wall, the most vulnerable one, whose faces on both sides are smooth, is the thickest (2.3 m [7.5 ft.]). In the middle, there was a stone triangular 'spur' dividing the fire lines of the battlements, two on each side. The lower part of the inner face of the wall has two large trapezoidal alcoves. These were probably used as embrasures for cannons, which however had never been used as there are no signs of any reconstruction work of the outer face of the wall.
Also the walls of the eastern part of the castle are not all the same. The eastern wall, 1.8 m (5.9 ft.) thick, is about 0.3 m (1 ft.) thicker than the remaining ones. The fact that it might have been used for defensive purposes would support the assumption that the castle originally did not have any defensive walls.
Between the extension connecting the two castle buildings and the defensive walls there was a parapet walk, supported by a brick arch and leading to a privy protruding towards the outer face of the walls. Judging by the traces visible on the wall, it can be assumed that the privy was constructed some time after the erection of the defensive walls. The castle entrance gate was located in the north, towards the lake. On this side, the way to the castle led through a bridge, the evidence of which are four pillars - two near the castle wall and the other two near the defensive wall. On the basis of an archeological research, it has been established that the castle had been destroyed during the Swedish invasion of 1656. It was later rebuilt by its successive owners in the 17th and 18th centuries.
The only preserved inspection documentation of 1772 reveals significant information. We can learn from it that the two castle buildings were reconstructed. However, the southern extension was no longer needed as a new entrance path led to the castle from the south (from the village of Gosławice). From the north, according to the documentation, a new building was about to be finished.
A presently-existing building stretching between the eastern defensive wall and the castle wall was probably constructed at the twilight of the 18th or at the beginning of the 19th century. Another building, situated between the eastern defensive wall and the castle, was built some time later for the purposes of the estate. It served as a maintenance complex and a servant house.
In the 19th century the castle's residential function gradually diminished. The last owner of Gosławice to reside in the castle was Melchior Łącki. In 1821 the Gosławice premises were sold to Klemens Kwilecki, who made them a gift to his son Hektor Kwilecki. He latter married Maria Izabella Tauffkirchen in 1830. They settled in a modest manor in the village of Maliniec. Later the manor underwent alterations - a few new rooms were added. In 1838 a construction of a granary situated between a church and a castle began.
The castle underwent a partial alteration to serve as a brewery. Between the First and Second World War it was nothing but ruins. After 1945 the building housed warehouses. The structures stretching between the defensive walls and the castle were occupied by farm workers.
After the administrative reform of 1975, the site aroused the interest of the Provincial Restorer for the Historic Sites and Monuments in Konin. Between 1978-1986 the castle was restored and adapted to become the seat of the District Museum, which at that time had no headquarters.
Lower parts of the two main buildings' gothic walls which withstood the test of time were prepared for the construction of the upper story. The project of the reconstructed elements was drafted on the basis of the original version. However, the authorities resigned from reconstructing the maintenance complex built at the end of the 19th century as it would shadow the view of the castle. The new fragments of the walls were plastered.
On November 30, 1986 the castle received its first visitors.