An extra gun platform was built at the foot of the tower in 1588 at the time of the Spanish Armada. It became a fort in 1643 during the English Civil War when a wall was built around it. In 1646 the Prince of Wales, later Charles II, visited it while he was staying at Elizabeth Castle. In 1651 despite having 12 cannon it surrendered to the Parliamentarian forces. In the 1730s the fort was remodelled as part of the same programme that was taking place at Elizabeth Castle. A century later the guns were mounted on traversing platforms and loopholes were cut into the walls. These changes meant that the size of the garrison had to be increased to one hundred men and three officers. This resulted in more barrack accommodation, extra water tanks and magazines being built. Following the Crimean War the fort fell into a rapid decline so that by 1880 the cannon had been withdrawn and the garrison consisted of one caretaker. The Germans refortified the whole of the islet during the Occupation.
How to get there.
This link shows you the location of the fort.
Some photography on this page is reproduced thanks to Chris Brookes.