The Rock is one of the most dramatic landforms in southern Europe. Most of its upper sections fall within the Upper Rock Nature Reserve. Tickets include admission to O'Hara's Battery on the Rock's summit, St Michael’s Cave, full of stalagmites and stalactites, the Apes’ Den, the Great Siege Tunnels hewn out of the Rock during the siege of 1779–83, the 1333 Moorish Castle, the Military Heritage Centre, Nelson's Anchorage and the spine-tingling Windsor Suspension Bridge and glass-floor, 340m-high Skywalk.
The WWII tunnels, where the Allied invasion of North Africa was planned, can also be visited, but you'll need to book ahead; you must have a nature-reserve ticket to access the tunnels, but they aren't actually included in that ticket. Even combined, the Great Siege and WWII tunnels constitute only a tiny proportion of the Rock's more than 50km of tunnels, most of which remain off limits.