The steep flanks of the hill made assaults on the castle difficult or, depending on the terrain, even impossible.
[2] Hilltop and spur castles were introduced by the Franks in order to hinder the deployment of heavy siege machinery.
[3] The classic example of a German hilltop castle is the 13th-century Otzberg, which comprises a circular bergfried on a hill above the village of the same name.
[4] The Cathars used a number of inaccessible hilltop castles as refuges, such as Château de Montségur which stands on the summit of a steep rocky mountain.
The rise of towns as economic and political centres reduced the value of such castles for trade and governance.