Short answers, pulled from the story.
A fortification is a military construction designed to defend territory in warfare and to establish rule in a region during peacetime. It is also called a fort, fortress, fastness, or stronghold. The word derives from the Latin fortis, meaning strong, and facere, meaning to make.
A castle is distinct from a generic fort or fortress because it is the 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.
Amnya Fort in western Siberia is described as one of the oldest known fortified settlements and the northernmost Stone Age fort. The walled town of Sesklo in Greece dates from 6800 BC, and Solnitsata near Provadia in Bulgaria dates from 4700 BC. Uruk in ancient Sumer is one of the world's oldest known walled cities.
Cannons arrived on the 14th-century battlefield and made medieval-style fortifications largely obsolete. Fortifications evolved into much lower structures with ditches and earth ramparts that could absorb and disperse cannon fire, since walls exposed to direct fire were very vulnerable. This led to star-shaped forts with bastions and hornworks, such as Fort Bourtange.
Advances in modern warfare since World War I made large-scale permanent fortifications obsolete in most situations. Escalating artillery and airpower could destroy almost any located target, and by 1950 nuclear weapons could destroy entire cities. Permanent fortifications could also be bypassed by mobile warfare, as happened with the Siegfried Line, the Stalin Line, and the Atlantic Wall.
Military airfields protect aircraft with revetments, hesco barriers, hardened aircraft shelters, and underground hangars, while munitions are kept in bunkers and bunds. At Bien Hoa in South Vietnam on the 16th of May 1965, a chain-reaction explosion destroyed 13 aircraft, killed 34 personnel, and injured over 100, which drove the construction of revetments and shelters across South Vietnam.
Fortification is usually divided into permanent fortification and field fortification, with an intermediate branch known as semipermanent fortification. The art of setting out a camp or constructing a fortification has been called castrametation since the time of the Roman legions.