St. Mawes Castle guards the eastern entrance to the estuary known as Carrick Roads. It is the companion of Pendennis and exactly contemporary. These two Henrician coastal forts offer some interesting contrasts. In each a squat round tower is the chief feature, but instead of having a square residential block slapped on in front of it, the St. Mawes tower is elaborated by three attached semi-circular bastions with parapets at a lower level. A distinctive stair turret caps the tower.
St. Mawes is unlike Pendennis but like the majority of Henry VIII's forts in being low lying and thus able to challenge enemy shipping at close quarters. Both castles share Henry's other fortifications, the rounded merlons designed to deflect cannon balls, the large embrasures for guns at several levels, and the emplacements for drawbridge and portcullis, the latter showing that the forts were intended to offer some resistance at close quarters if the enemy ever landed. Above the entrance we encounter again a panel of the royal arms. On the rocks in front of the castle is a semi-circular blockhouse matching the one in Pendennis, perhaps erected as an emergency fortification before the real work started.
In terms of size, the castles would appear to have been conceived as equals and their early governors were bitter rivals. With the Elizabethan enlargement of Pndennis, however, St. Mawes shrank into a subsidiary role. Its part in the Civil War typifies this. In contrast with Pendennis Castle's heroic stance, the royalist governor here wisely judged the castle to be indefensible from the land and surrendered without a shot being fired. The insignificance of St. Mawes has allowed it to survive in a very unspoiled condition. Not only has the stonework suffered very little, but within there is a surprising amount of original woodwork.