Another improvement from the 12th century onwards was the creation of shell keeps, involving replacing the wooden keep on the motte with a circular stone wall. Buildings could be built around the inside of the shell, producing a small inner courtyard. Restormel Castle is a classic example of this development with a perfectly circular wall and a square entrance tower while the later Launceston Castle, although more ovoid than circular, is another good example of the design and one of the most formidable castles of the period. Round castles were unusually popular throughout Cornwall and Devon. Although the circular design held military advantages, these only really mattered in the 13th century onwards; the origins of 12th-century circular design were the circular design of the mottes; indeed, some designs were less than circular in order to accommodate irregular mottes, such as that at Windsor Castle.