St. George's Hall, Rock of Gibraltar by Anita363

Turns out the Rock is honeycombed with >50 km of tunnels. Most of those date to WW II, when the British constructed facilities that could house a garrison of 16,000 men for a year completely inside the Rock. But the first tunnel, much of the way across the north face of the Rock, dates to the Great Siege of 1779–1783, the 14th and final attempt by Spanish and French forces to retake Gibraltar from the British. To cover a blind spot in the approach by land, the Brits needed to get cannon to a nose along the sheer cliff face. Rather than attempting to haul a gun up to the top of the Rock (altitude 1300 ft, or 400 m) and then lower it halfway down the sheer cliff face, they undertook an only slightly less heroic tunneling effort. And along the way they discovered that vents opened laterally to the rockface made dandy embrasures for gun emplacements, so they ended up installing over a dozen guns inside the rock (several of them in a large gallery inside the target nose, seen here), instead of a lone one on the intended perch outside.