The 400-m-high Rock of Gibraltar is a partly overturned klippe of Early Jurassic dolomitic limestone, notched by raised shorelines and flanked by Quaternary scree breccias and windblown sands. It dominates a narrow 5-km-long peninsula jutting south from Spain at the western entrance to the Mediterranean Sea. Fortified from at least 1160 to World War II successively by the Moors, Spanish, and British, and subjected to 15 major sieges between 1309 and the Cold War of 1947 to 1989, Gibraltar is arguably one of the most densely fortified and fought over places in Europe. Stone walls, bastions, and numerous artillery positions built to enhance the natural defenses of the coastal cliffs now provide a tourist attraction, but constrain development of the modern city. Occasional rockfalls and the need for slope-safety measures are continuing concerns, especially in areas of scree breccia quarried to provide fill for the extension of a Royal Navy harbor between 1893 and 1905 and a Royal Air Force airfield largely between 1941 and 1943. Water supply has posed a problem throughout the history of the fortress, leading to innovative development of rainwater catchment areas on natural slopes, dual potable/sanitary water supplies, and projects to enhance these by cloud condensation or groundwater abstraction, before near total commitment to desalination from 1993. Tunnels and underground chambers are major features of the Rock. Mostly excavated in five phases between 1782 and 1968 to provide access, reservoirs, accommodation, or storage, they now total over 50 km in length, generally unlined. Tunnel integrity is dependent on excavation technique and bedrock characteristics.

From the early eighteenth century to recent years, much of the major construction work on Gibraltar was directed and often carried out by Royal Engineers. In 1994, disbandment of 1st (Fortress) Specialist Team RE brought over two centuries of British military engineering on the Rock to an end—and provided a legacy of works and land now largely inherited by civilian bodies responsible to the Gibraltar government.