Until the early 16th century the west of Cornwall remained undefended, but an incident in the Fal estuary between a Spanish fleet and a group of French warships caused the local gentry to press King Henry VIII for some protection against any future invasions. In 1538 two blockhouses were built on opposite headlands overlooking Falmouth Bay at Pendennis Point and at St Mawes, but these were enhanced just two years later with the addition of proper castle structures.

Taking some five years, and £5,000 to complete, St Mawes Castle consists of a central circular keep with three semi-circular bastions looking out to sea. It was a typical Henry VIII artillery fort, a low round building with massively thick walls that provided a difficult target for enemy ships, and purpose-built to mount canons on each platform. During Tudor times even the roof was used as a gun platform, and the small domed turret was a look-out point. Despite its defensive strength from a sea attack, St Mawes Castle did not have a commanding landward position. Built at the foot of a hill it was always going to be vulnerable from a land attack, and this was proved during the Civil War when the royalist governor of St Mawes Castle surrendered without a shot being fired. Even before then its counterpart at Pendennis was deemed the more important of the two castles, receiving a costly Elizabethan enlargement, and being given additional guns from St Mawes.

Even though St Mawes Castle was continually occupied by military forces until well after the end of the Second World War, it has survived in a remarkably unspoilt condition and this is largely due to the fact that it was considered less significant than Pendennis Castle. The external stonework has suffered little, and few modifications have been made to the original castle but the Grand Sea Battery has been updated over the centuries to cope with the changing demands of modern gun power. One of the most surprising features at St Mawes Castle is the amount and quality of carving throughout - most of Henry's coastal forts were solid, utilitarian outposts designed primarily to ward off attackers, not to display the lavish wealth and power of the King.

Today a walk around St Mawes Castle is like stepping back in time. Now very peaceful, and commanding a wonderful view across the picturesque bay, it is not difficult to visualise this 'homely' castle being occupied by soldiers from the 16th to the 20th centuries.