The medieval castle was built of bricks on a stone foundation, on a mound protruding from a small island on the lake. The waters of the lake provided protection from the west, while from the south there was a boggy and rough area. The remaining sides were protected by an irrigated moat.
The castle had the form of an irregular quadrangle (in particular, the south curtain of the wall was curved) measuring approximately 22.3 x 24.8 x 31 x 32 meters, with a cylindrical main tower with a diameter of 8 meters in the north-east corner. It controlled the entrance gate to the courtyard and, at the same time, the road from Poznań to Wrocław passing through Kórnik. Leading from the north, the entrance was reinforced with a gatehouse with dimensions of 6.5×8.5 meters, protruding from the perimeter of the walls. The residential building was a house with a length of 10 and a width of 9 meters, perhaps two or three-storey. Probably like other buildings of this type in the Middle Ages, it did not have internal divisions into smaller rooms. It was probably located in the eastern part of the courtyard, as the western one was wetter (located closer to the lake) and used only during the development in the fifteenth century. Its warming could have been provided by a tiled stove, as tile fragments with a tracery ornament and a fish bladder motif were found.
After the rebuilding from the first half of the 15th century, two half-timbered houses were built (on a stone foundation), three-story, with dimensions of 5.5 x 11 meters, which formed the shape of the letter L. Inside, the individual storeys separated wooden, flat ceilings. Near the houses, two projections in the forms of semi-cylindrical towers were created (the diameter of one of them was 3 meters). All these new constructions were covered with shingle roofs, and the western buildings according to the contract were to receive a kind of decorative crenellation. A wooden attic was also added to one of the pre-existing buildings, perhaps the main brick dwelling house. The new storey was to be intended for a grain warehouse. Master Nicholas also received orders for some works in the upper parts of the gatehouse, but their goal is not entirely clear. It is known, however, that a new drawbridge and portcullis were made at that time.
