Following the end of World War One, St. Mawes Castle was handed over from the military to the Department of Works. It was landscaped and opened to the public. However, when war came again, in 1939, the military reoccupied the site.

On Monday 5 August 1940 construction began on a gun emplacement and director tower just to the North West of the castle, along what is now Castle Drive. A Nissen hut was also built alongside the battery and control tower. The gun battery housed twin six pounder quick firing guns intended for defence against German E-boats. The battery, tower and hut can be clearly seen in the below photo, taken in 1947.

On Saturday 16 August 1941, the garrison to man the now completed defences arrived. This consisted of three officers and 112 other ranks belonging to No. 173 Coast Battery. They were accommodated in the castle itself, the St. Mawes Hotel and a private residence, the Haven.

To support the battery four searchlights were installed. These were positioned below the castle just above the waterline. They were also camouflaged. The four rectangular structures that housed the searchlights are clearly visible in the 1947 photo below. The power for these was supplied by a generator housed in the engine room that was built in 1902. The oil store for the generator was located where the car park is today and can be seen in the photos.

Anti-aircraft defence was provided by a 40mm Bofors gun sited on the terrace where there is now a line of cannon. There was also a barrage balloon in the field above the castle; its position is marked on the Luftwaffe aerial photograph below.

In addition to the primary defences detailed above various other measures were in place:

The gun emplacements dating from 1904, above the castle, were not used. However, the underground magazine built to supply them was utilised as a training venue. A spigot mortar was sited along the road into St. Mawes. Barbed wire was placed around the castle and two roadblocks placed on the road either side of the castle. The locations of the roadblocks and barbed wire can be seen on the plan below, which is on display at Pendennis.

The observation post, the building visible in the photo below on the far side of the field above the castle, was utilised by the Royal Navy.

The military finally vacated the castle in 1956.