The oldest preserved walls of the castle in Koźmin are connected with the castellan of Nakło and the voivode of Poznań, Bartosz Wezenborg of Odolanów, who erected it in the second half of the 14th century. In the 16th century, the renaissance reconstruction of the stronghold was carried out by Andrzej II Górka. From 1621, the castle belonged to the Przyjemski family, who re-converted the residence. At the beginning of the 18th century they gave it to Sapieh family. The castle was finally rebuilt in the second half of the 19th century.
The brick castle was founded on a rectangular plan with reinforced three buttresses at the corners and a small tower in the north-west corner. At the eastern section of the northern wall, a gate tower was erected. The internal appearance was created by a two-story house on the L-shaped plan with a north wing with basement. In the fifteenth century north-west tower was strengthened, and foregate was added to the gatehouse, flanked by two cylindrical towers with a diameter of 6 meters. The northern wing was also expanded. In the 16th century, Andrzej Górka built up the north-west tower. and he erected the eastern house.
Nowadays, in Koźmin castle its medieval roots are hardly visible. The only visible remnant of the gothic stronghold is the brick, octagonal tower. The building is the seat of the school complex.
Leksykon zamków w Polsce, L.Kajzer, S.Kołodziejski, J.Salm, Warszawa 2003.