On the Lenne River in the western part of the Sauerland. A Medieval hill castle erected by the early Counts of Berg, around 1108. Capital of the County of Mark until moved north to Hamm in the 1220s. The County of Mark extended between the Lippe and Agger Rivers (north-south) and between Gelsenkirchen and Bad Sassendorf (west-east). The east-west flowing Ruhr separated the county in two different regions: the northern, fertile lowlands of Hellweg-Börde; and the southern hills of the Süderbergland (Sauerland). In the south-north direction the southern part of the county was crossed by the Lenne. In the region of the Lower Lenne was the County of Limburg (1243–1808), a fiefdom of Berg.