After the eleventh century, as masonry techniques advanced, ashlar blocks became more rectangular, resulting in structurally more stable walls that could incorporate more refined architectural moulding and detailing that can be seen in corbelling, buttressing, lintels and arching. At the same time there was increasing influences from English and continental European designs, such as the Romanesque chevron pattern detailing on the piers in the nave of Dunfermline Abbey (1130–40), which were modelled on details from Durham Cathedral, and the thirteenth century East-end of Elgin Cathedral, which incorporated typical European Gothic mouldings and tracery. In the fifteenth century continental builders are known to have been working in Scotland. French master-mason John Morrow was employed at the building of Glasgow Cathedral and the rebuilding of Melrose Abbey, both considered fine examples of Gothic architecture. The interiors of churches were often elaborate before the Reformation, with highly decorated sacrament houses, like the ones surviving at Deskford and Kinkell. The carvings at Rosslyn Chapel, created in the mid-fifteenth century, elaborately depicting the progression of the seven deadly sins, are considered some of the finest in the Gothic style. Late Medieval Scottish churches also often contained elaborate burial monuments, like the Douglas tombs in the town of Douglas. The early sixteenth century saw crown steeples built on churches with royal connections, symbolising imperial monarchy, as at St. Giles Cathedral, Edinburgh. Scotland is known for its dramatically placed castles, many of which date from the late medieval era. Castles, in the sense of a fortified residence of a lord or noble, arrived in Scotland as part of David I's encouragement of Norman and French nobles to settle with feudal tenures, particularly in the south and east, and were a way of controlling the contested lowlands. 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", with a high embattled curtain wall.