Fortifications are military constructions or buildings designed for the defense of territories in warfare, and also used to solidify rule in a region during peace time. Humans have constructed defensive works for many thousands of years, in a variety of increasingly complex designs. The term is derived from the Latin fortis ("strong") and facere ("to make").From very early history to modern times, walls have been a necessity for cities to survive in an ever changing world of invasion and conquest. Some settlements in the Indus Valley Civilization were the first cities (They were very small in comparison to later, major cities) to be fortified. In ancient Greece, large stone walls had been built in Mycenaean Greece, such as the ancient site of Mycenae (famous for the huge stone blocks of its 'cyclopean' walls). A Phrourion was a fortified collection of buildings being used as a military garrison and is the equivalent of the Roman castellum or English fortress. These construction mainly served the purpose of a watch tower, to guard certain roads, passes, and lands detrimental to the kingdom. Though smaller than a real fortress, these constructions acted as a border guard rather than a real strongpoint to watch and maintain the border.The art of setting out a military camp or constructing a fortification traditionally has been called "castramentation" since the time of the Roman legions. Fortification is usually divided into two branches: permanent fortification and field fortification. There is also an intermediate branch known as semi-permanent fortification. Castles are fortifications which are regarded as being distinct from the generic fort or fortress in that it describes a residence of a monarch or noble and commands a specific defensive territory.Roman forts and hill forts were the main antecedents of castles in Europe, which emerged in the 9th century in the Carolingian Empire.The Early Middle Ages saw the creation of some towns built around castles. Medieval-style fortifications were largely made obsolete by the arrival of cannons on the 14th century. Fortifications in the age of black powder evolved into much lower structures with greater use of ditches and earth ramparts that would absorb and disperse the energy of cannon fire. Walls exposed to direct cannon fire were very vulnerable, so were sunk into ditches fronted by earth slopes.The arrival of explosive shells in the 19th century led to yet another stage in the evolution of fortification. Star forts did not fare well against the effects of high explosive and the intricate arrangements of bastions, flanking batteries and the carefully constructed lines of fire for the defending cannon could be rapidly disrupted by explosive shells. Steel-and-concrete fortifications were common during the 19th and early 20th centuries. However the advances in modern warfare since World War I have made large-scale fortifications obsolete in most situations.Demilitarized zones along borders are arguably another type of fortification, although a passive kind, providing a buffer between potentially hostile militaries.