Calenberg Castle (German: Burg Calenberg, later called Schloss Calenberg and Feste Calenberg; ruins known as Alt Calenberg) was a medieval lowland castle in central Germany, near Schulenburg in the borough of Pattensen, 13 km west of the city of Hildesheim. It was built as a water castle in 1292 by the Welf duke, Otto the Strict, in der Leine river meadows between 2 branches of the Leine river on the southern part of the chalk marl hill of the Calenberg. At the start of the 16th century it was converted into a fort (Feste). In the 15th century, Fort Calenberg gave its name to the Welf Principality of Calenberg. Following the Thirty Years' War it lost its military importance and was slighted. Today it is a ruin with underground vaults that are surrounded by high ramparts.