The positioning of castles was influenced by the available terrain. Whereas hill castles such as Marksburg were common in Germany, where 66 per cent of all known medieval were highland area while 34 per cent were on low-lying land, they formed a minority of sites in England. Because of the range of functions they had to fulfil, castles were built in a variety of locations. Multiple factors were considered when choosing a site, balancing between the need for a defendable position with other considerations such as proximity to resources. For instance many castles are located near Roman roads, which remained important transport routes in the Middle Ages, or could lead to the alteration or creation of new road systems in the area. Where available it was common to exploit pre-existing defences such as building with a Roman fort or the ramparts of an Iron Age hillfort. A prominent site that overlooked the surrounding area and offered some natural defences may also have been chosen because its visibility made it a symbol of power. Urban castles were particularly important in controlling centres of population and production, especially with an invading force, for instance in the aftermath of the Norman Conquest of England in the 11th century the majority of royal castles were built in or near towns.