Stora Sundby Castle, Castle in Eskilstuna Municipality, Sweden

Stora Sundby is a castle whose present silhouette dates from the 1831–1848 rebuilding by Count Carl De Geer, designed by Scottish architect Peter Frederick Robinson and built by Abraham Nyström. Its four large towers, twelve small towers, 52 rooms and 365 windows are arranged to mirror the calendar. Inside, the Knight's Hall has a black-and-white painted board floor, a ceiling painted as baroque stucco with Axel Sparre's coat of arms, and seven chandeliers carved with elk and deer heads and real antler crowns.

A fortified farm with a stone tower stood here by 1364; its ruin lies about 300 metres north of the present building. Lars Siggesson Sparre built the first castle, completed with three towers in 1584, and his son Erik Larsson Sparre added two corner towers and a stair tower. Axel Sparre had Nicodemus Tessin the Elder modernise it around 1660–1670, and Erik Sparre added the fourth tower in the early 1700s. Count Carl De Geer rebuilt it in Norman style from 1831 to 1848.

The castle's silhouette encodes a calendar: four large towers for the seasons, twelve small towers for the months, 52 rooms for the weeks and 365 windows for the days. The Knight's Hall ceiling shows Axel Sparre's coat of arms, and seven chandeliers of carved elk and deer heads with real antlers hang above the painted checkerboard floor.

The castle is a private residence of the Klingspor family, who have owned it since 1888, and it is not open as a regular museum. The castle and about ten surrounding buildings were declared a private listed building complex in September 1983. The grounds lie by Hjälmaren near Alberga in Öja parish, about 300 metres south of the old castle rock, where wall remains of a 13th-century grey-stone tower were excavated in 1960.

The Knight's Hall ceiling holds seven chandeliers carved from wood with elk and deer heads and real antler crowns, a type now very rare in Sweden. The hall's black-and-white painted board floor imitates marble, and its ceiling painting shows Axel Sparre's coat of arms.