The castle has a central circular tower with three lower semi-circular bastions that give the castle the appearance of a clover leaf when viewed from above. The main entrance is on the landward side, protected by a rock-cut ditch. Cannon could be mounted on the roof of the main tower and bastions and on floors within the bastions. The defences at St Mawes were designed to counter the threat of an attack from the sea, but overlooked by high ground it was vulnerable to an attack from the land. The more defensible Pendennis Castle became the dominant fortress in the region and as a result St Mawes did not see the major alterations that took place at Pendennis and has retained most of its original Tudor design.
When the castle came under attack in 1646, during the Civil War, the governor of St Mawes surrendered without a shot being fired, realising it was hopeless to try and defend against an attack from the land. However it continued to serve a useful purpose as a gun emplacement protecting the harbour at Falmouth until as recently as the Second World War.