Defenses have been constructed for thousands of years. Bronze and Iron Age hillforts took advantage of natural hills for defense purposes, and the Romans built the Saxon Shore Forts along the southeast coast of Britain to deter invasion.
The word fort is derived from the Latin fortis meaning "strong" and many military installations are known as forts. The term fortification also refers to improving other defenses such as city walls. Permanent fortifications were built of enduring materials, but field fortifications needed little preparation, using earth, timber, or sandbags.
The arrival of cannons in the fourteenth century made medieval fortifications obsolete. Later constructions included ditches and earth ramparts to absorb the energy of cannon fire. Explosive shells in the nineteenth century led to a further evolution; the profile of the fort became lower, surrounded by an open sloping area that eliminated cover for the enemy. The fort's entry point was a gatehouse in the inner face of the ditch, with access via a bridge that could be withdrawn. Most of the fort was built underground, with passages connecting blockhouses and firing points. Guns mounted in open emplacements were protected by heavy parapets. Offensive and defensive tactics became focused on mobility. In the twentieth century defending tanks were concentrated in mobile units behind the line. If an offensive was launched, reinforcements could be sent to that area.
Reinforced concrete fortifications were common during the nineteenth and early twentieth centuries, but modern warfare has made large-scale fortifications obsolete; now, only deep underground bunkers provide sufficient protection. Today, forts mostly survive as popular tourist destinations.
