These were primarily wooden motte-and-bailey constructions, of a raised mount or motte, surmounted by a wooden tower and a larger adjacent enclosure or bailey, both usually surrounded by a fosse (a ditch) and palisade, and connected by a wooden bridge. They varied in size from the very large, such as the Bass of Inverurie, to more modest designs like Balmaclellan. In England many of these constructions were converted into stone "keep-and-bailey" castles in the twelfth century, but in Scotland most of those that were in continued occupation became stone castles of "enceinte" from the thirteenth century, with a high embattled curtain wall. The need for thick and high walls for defence forced the use of economic building methods, often continuing the Scottish tradition of dry-stone rubble building, which were then covered with a lime render, or harled for weatherproofing and a uniform appearance. In addition to the baronial castles there were royal castles, often larger and providing defence, lodging for the itinerant Scottish court and a local administrative centre. By 1200 these included fortifications at Ayr and Berwick. In Scotland Alexander II (r. 1198–1249) and Alexander III (1241–86) undertook a number of castle building projects in the modern style. Alexander III's early death sparked conflict in Scotland and English intervention under Edward I in 1296. The resulting Wars of Independence brought this phase of castle building to an end and began a new phase of siege warfare.