A bank and ditch enclosure was a simple form of defence, and when found without an associated motte is called a ringwork; when the site was in use for a prolonged period, it was sometimes replaced by a more complex structure or enhanced by the addition of a stone curtain wall. Building the hall in stone did not necessarily make it immune to fire as it still had windows and a wooden door. This led to the elevation of windows to the second storey – to make it harder to throw objects in – and to move the entrance from ground level to the second storey. These features are seen in many surviving castle keeps, which were the more sophisticated version of halls. Castles were not just defensive sites but also enhanced a lord's control over his lands. They allowed the garrison to control the surrounding area, and formed a centre of administration, providing the lord with a place to hold court.