Fortifying our coast against invaders has been a challenge since ancient times. A visit to the Essex coast will show relics that range from the Roman fort of Othona at Bradwell (represented now by a few foundations of walls and the Saxon chapel built on the old gatehouse) right through to World War 2 pill boxes.

In spite of the Royal Navy dominating the seas, the coast was seen as vulnerable during the long-drawn-out Napoleonic Wars, particularly in the 1800s.

In 1803 a chain of square gun-towers was proposed, but the plan changed radically thanks to study of a tricky attack the Navy had launched on a fort at Mortella in Corsica in 1794. This circular fort with thick stone walls had only been armed with three guns, but it had repulsed attacks by a 74-gun battleship and a 32-gun frigate, causing sixty casualties. It took two days of bombardment by soldiers who placed their guns on land 150 yards from the tower to force a surrender.

Brigadier General Twiss proposed 58 such towers (named Martello Towers) to protect vulnerable beaches and harbours around the South and East coasts. Typically they were forty feet high, with eight foot thick walls. The open top floor had one to three guns on traversing carriages, the middle floor had accommodation for 24 men, and the ground floor had the magazine for powder and shot.

They were designed to repel enemy warships and to easily sink invasion barges. If enemy troops were landed, it was estimated that the forts could hold out long enough for local troops to assemble and support them.

29 Martello Towers were built along the East Coast between 1808 and 1812. These were larger and stronger than the earlier South Coast towers, each being built of 750,000 bricks laid in an interlocking pattern to give maximum strength.

Eleven on these towers were built in Essex, mostly along the long vulnerable stretch of beach running past Clacton. Six of these towers survive: Tower A at St Osyth, Tower C at Jaywick, Towers D, E and F at Clacton-on-Sea, and Tower K at Walton on the Naze.

The military use of the towers did not end in the peace of 1815, and some even served in World War II as observation points.

Because of their sturdy construction, the towers look very much as they did when built, but their interiors have had a chequered history – at various times they have accommodated a bar and discotheque, a museum, an art gallery, a children's zoo and a storeroom.