In the design of many castles he used a concentric plan where a succession of nested walls and towers were built, the walls getting taller towards the centre of the castle. In these castles there are no central keeps, but one or more of the wall towers are large enough to be used in the same way. The gatehouses were also very strong. In the early years of the fourteenth century James was employed in Scotland building castles there as well. His works include Linlithgow Peel, a fortification which replaced a royal manor.