Flak towerFlak towers (Flaktürme) were large, above-ground, anti-aircraft gun blockhouse towers constructed by Nazi Germany. There were 8 flak tower complexes in the cities of Berlin (three), Hamburg (two), and Vienna (three) from 1940 onwards. Other cities that used flak towers included Stuttgart and Frankfurt. Smaller single-purpose flak towers were built at key outlying German strongpoints, such as at Angers in France, and Heligoland in Germany. The towers were operated by the Luftwaffe to defend against Allied strategic air raids against these cities during World War II.
Pillbox (military)A pillbox is a type of blockhouse, or concrete dug-in guard-post, often camouflaged, normally equipped with loopholes through which defenders can fire weapons. It is in effect a trench firing step, hardened to protect against small-arms fire and grenades, and raised to improve the field of fire. The modern concrete pillbox originated on the Western Front of World War I, in the German Army in 1916. The origin of the term is disputed.
Air raid shelterAir raid shelters are structures for the protection of non-combatants as well as combatants against enemy attacks from the air. They are similar to bunkers in many regards, although they are not designed to defend against ground attack (but many have been used as defensive structures in such situations). During World War II, many types of structures were used as air raid shelters, such as cellars, Hochbunkers (in Germany), basements, and underpasses. Bombing raids during World War I led the UK to build 80 specially adapted London Underground stations as shelters.
BunkerA bunker is a defensive military fortification designed to protect people and valued materials from falling bombs, artillery, or other attacks. Bunkers are almost always underground, in contrast to blockhouses which are mostly above ground. They were used extensively in World War I, World War II, and the Cold War for weapons facilities, command and control centers, and storage facilities. Bunkers can also be used as protection from tornadoes. Trench bunkers are small concrete structures, partly dug into the ground.
FortificationA fortification is a military construction designed for the defense of territories in warfare, and is used to establish rule in a region during peacetime. The term is derived from Latin fortis ("strong") and facere ("to make"). From very early history to modern times, defensive walls have often been necessary for cities to survive in an ever-changing world of invasion and conquest. Some settlements in the Indus Valley Civilization were the first small cities to be fortified.
Maginot LineThe Maginot Line (Ligne Maginot, liɲ maʒino), named after the French Minister of War André Maginot, is a line of concrete fortifications, obstacles and weapon installations built by France in the 1930s to deter invasion by Germany and force them to move around the fortifications. The Maginot Line was impervious to most forms of attack. Consequently, the Germans invaded through the Low Countries in 1940, passing it to the north.