St Mawes Castle is one of the earlier artillery fortifications dating back to 1542.
In the 1860 the Castle was armed with eight 56-pdr SB shell guns and four 64-pdr RML guns (SB conversions?). By 1880 the 56-pdr SB guns had been removed. The four 64-pdr guns at St Mawes Castle were supported by four similar guns at Pendennis Castle and another two at Crab Quay.
By about 1888 the decision had been made to establish a Submarine Minefield to protect Flamouth Harbour, with the mine laying to be done by the Falmouth Volunteer Submarine Miners, R.E. They initially number sixty men as established an depot at Arwenack House. (The Falmouth Packet Newspaper). The Minefield was composed of twelve lines of mines, including both controlled and automatic mines. The minefield required observation post at both Pendennis Castle and St Mawes Castle as well as Quick Fire Batteries for protection. Defence Electric Lights provided support during the hours of darkness.
To protect the minefield two 6-pdr QF guns and two Maxim Machine guns were mounted in the old sea battery by 1898. This 6-pdr Battery was found to be to low, only 12.04 metres above the sea. A new 12-pdr Battery was therefore built to the rear of St Mawes Castle at 110 Metres. Two 12-pdr guns were installed in this new battery by 1901. In 1903 the two 6-pdr guns were removed from Pendennis Castle, and an additional two 12-pdr guns were mounted in the upper battery by 1904.