The size of these castles varied depending on the geography of the site, the decisions of the builder and the available resources. Analysis of the size of mottes has shown some distinctive regional variation; East Anglia, for example, saw much larger mottes being built than the Midlands or London. While motte-and-bailey and ring-work castles took great effort to build, they required relatively few skilled craftsmen allowing them to be raised using forced labour from the local estates; this, in addition to the speed with which they could be built – a single season, made them particularly attractive immediately after the conquest. The larger earthworks, particularly mottes, required an exponentially greater quantity of manpower than their smaller equivalents and consequently tended to be either royal, or belong to the most powerful barons who could muster the required construction effort. Despite motte-and-bailey and ringworks being common designs amongst Norman castles, each fortification was slightly different – some castles were designed with two baileys attached to a single motte, and some ring works were built with additional towers added on; yet other castles were built as ringworks and later converted to motte-and-bailey structures.