The castle was built at the end of the 14th century on the initiative of the Nałęcz family. Maćko Borkowic, the Poznań voivode or his daughter Wichna, most probably began the process, and her grandson Wincenty continued it. In the middle of the 15th century Sieraków received Łukasz Górka coat of arms Łodzia, who rebuilt the castle. In 1571 it was taken over by the starost Jakub Rokossowski, and in 1591 Sieraków bought the castellan Jan of Bnin Opaliński, who raised the castle houses and converted them into a baroque residence. In 1763 castle and estate were bought by the baron Piotr Mikołaj Neugarten von Gartenberg, using the Polish name Sadogórski, and probably during his time demolition of the northern wing was carried, leaving only the southern wing. In 1829 in relation with the construction of a new road, the remains of the castle were demolished. Only the south wing survived. In 1991 it was decided to restore the remains of the castle and to put in it the tombs of the Opalinsk family. Construction work lasted two years. In the lack of the sources of the castle’s appearance, it was decided to reconstruct the southern range only.
The castle was east of the town and was separated from it by a moat. It stood on a regular, artificial mound. Its defenses was increased by the river, surrounding from the south. The earliest phase is connected with the emergence of brick curtain walls with the gate from the east. From the south, a residential range, initially timber or half-timber framed, was located. In the 15th century a brick north range with a width of 8 meters was erected with characteristic corner buttreses. At the same time, or slightly later, a new southern building of similar dimensions was constructed. Another redevelopment introduced new partition walls and a shorter west range connector.
Today, in the rebuilt south range of the castle, there is a museum which expositions present the history of the Sieraków Region from the earliest to the present. Particular attention should be paid to the tombs of the Opalińsk family, discovered in 1991 in the crypt of the church of St. Bernard. The castle is open: Tuesday – Friday 8.30-15.30, Saturday 11.00 – 15.00, Sunday 13.00 – 17.00, on Monday the museum is closed.
Leksykon zamków w Polsce, L.Kajzer, S.Kołodziejski, J.Salm, Warszawa 2003.