Great Siege Tunnels
Formerly known as the Upper Galleries, these tunnels were carved out during the Great Siege of 1779-82. At the northern end of Old Queen's Road, you can plainly see the openings from which the guns were pointed at the Spanish invaders. They form part of what is arguably the most impressive defense system anywhere in the world. The privately managed World War II tunnels nearby (prebooking essential via email) are also open to the public but are less dramatic.
