St Mawes artillery castle was built between 1540 and 1543, as one of a chain of coastal defences constructed by Henry VIII to counter the threat of French and Spanish invasion following the English Reformation. It is situated on a broad headland at the mouth of the River Fal on the south coast of Cornwall. St Mawes was built just above an earlier fortification dating to the late 1530s, which was probably a preliminary attempt to fortify this coastline. From the 17th to 20th centuries, the slopes of the headland around the artillery castle were modified by successive phases of gun batteries as well as World War II searchlight emplacements. A lower gun battery was also built just below the castle (see associated records).
The Henrician castle consists of a central keep with three semi-circular bastions arranged about it in clover-leaf pattern. The main entrance is on the landward side of the keep, approached through a hexagonal guardhouse and then by a stone bridge over a deep dry moat. Originally there may have been a drawbridge. The keep is of three storeys and housed a kitchen in the basement, accommodation for ordinary soldiers on the first floor, accommodation for the governor and chief officers on the second floor and an open gun platform on the roof reached by a winding staircase. In the late 17th century a lead-covered dome, called a cupola, was added to the castle's stair turret as a daymark - a maritime navigational aid. The part-circular forward bastion provided the castle's main gun deck on the ground floor and a raised gun platform on the upper floor.
The castle was built under the direction of Thomas Treffry and it is constructed of local slatestone rubble with granite employed on many of the architectural features. There are many carved and incised decorative elements to the design including the Royal Arms over the keep, carved sea monsters, shield plaques, gargoyles as well as dedicatory and laudatory inscriptions to Henry VIII.