Castles. Fortresses, residences of feudal overlords, romantic ruins and haunts for ghosts... They hold out imagination, they show up in historical fiction and fantasy alike. This page, however, is a Useful Notes page and is dedicated to castles in Real Life, their history and types. The first section will describe the castles of Western Europe, which are the first that spring to mind when we hear the word castle.
Castles evolved in, surprise, Western Europe during the Early Middle Ages. The original castles were fortified manor houses of various nobles that ruled the many shards of the shattered empire of Charlemagne.note Note that by "manor house," we mean "house big enough to house the lord, his servants, and his animals. These were generally heated by a single large fire and far from fancy—unless your definition of "fancy" is "made from halfway decent materials"—before the invention of the chimney in the 11th or 12th century made more complex arrangements practicable in these more northerly climates.) From these manors the feudal lords administered their lands, and they surrounded them with more and more fortifications to protect themselves from their enemies. Thus, the castles grew from lone towers surrounded by earthen berms to massive, concentric structures that could hold any siege.
Types of castles by construction
Motte and bailey. The most primitive kind of castle, the motte and bailey is barely above a pre-medieval hill fort. It is usually just a tower or a fortified manor standing on a hill, which may be a natural hill dug over with artificial trenches and berms or a wholly manmade mound (the motte). The motte was also outfitted with extra defences such as a wooden stockade (the bailey). Often the bailey was located sideways of the motte and did not encapsulate it; the steep slopes of the motte made walls unnecessary. Note that not all castles built on mottes are motte and bailey castles: the central section of the famous Windsor Castle, which is far from being "just a tower", has a large motte under it.
Keep and curtain wall. To improve the ability of a motte and bailey castle to withstand sieges, medieval engineers went for the most obvious decision: build a solid wall instead of a wooden stockade around the motte. They built massive walls, high enough to be unscalable without proper siege ladders, and later augmented those walls with towers. The tower also became more fortified and turned into a keep, or donjon - the main tower or fort of the castle, a smaller fortress inside it. Even when enemies breached or otherwise surmounted the curtain wall of the castle, the keep was able to fend them off for a while.
Moat. To add another layer of impenetrability to the curtain wall, medieval engineers augmented it with a deep ditch, or moat, around it. The purpose of the moat was to stop the attackers from breaking castle walls with battering rams and make it harder to use siege ladders. The moat was also often filled with water to stop undermining (digging under walls to make them collapse). A drawbridge or a permanent bridge could be used to cross the moat and reach the gate.
Gatehouse. Because of the moat, the castle gate became the prime target for attempts to breach the walls and break in. So the gate naturally became more fortified, built into a large, wide tower: the gatehouse. A typical gatehouse contained a lot of security measures to make battering the gate harder: corridors, portcullises, arrow slits overlooking the bridge.
Barbican. A barbican is another fortification built to protect the gate: a second, smaller gatehouse in front of it, connected to the main one with a pair of walls.
Enceinte. An enceinte is the motte, Mk.II, now made of stone! It's an inner solid stone wall surrounding the keep, making it a castle within a castle.
Concentric castle. Combining all of the above defensive measures resulted in a complex, many-layered castle with two or more sets of curtain walls and a keep surrounded with an enceinte. Such castles were built during the Late Middle Ages.
Quadrangular castle. A late development in castle building, this style does away with the keep and turns the curtain wall into a large rectangular building with a courtyard. In essence, the curtain wall is used as the outer wall of the building.
After gunpowder artillery became the main weapon of sieges, castle architecture entered into decline. Low-profile and complex structures were needed to resist artillery bombardments, and castles made way for bastions, star forts and similar fortified structires. However, during the period of Romanticism and Gothic literature in the XIX century, interest in castles renewed. These "revival castles" served no defensive function and were just stylized stately homes; the most famous example of such a castle is Neuschwanstein in Bavaria, Germany.
Types of castles by location
City castle. Found in the historical centre of a medieval city, and often as the core part of a larger fortification called a citadel, the urban castle houses the ruler of said city, city-state or realm of which this city is a capital.
Rural castle. Built as a standalone structure, this type of castle was more widespread than the urban castle in Western Europe. Often castles planned as standalone structures attracted folks from nearby villages and grew towns around them, eventually becoming city castles and citadels.
Castles and similar structures in other parts of the world
Central and parts of Eastern Europe. Here castles weren't much different from the Western European ones. The only difference is that the local castle-building tradition was not so old; there are no Early Medieval castles in Eastern Europe, and the oldest were built during the High Middle Ages.
Russia. This country's tradition of fortification is a bit different; the rural castle never caught on here. Urban castles and citadels, called kremlins, on the other hand, were quite widespread. Yes, kremlin is not the name of one specific famous citadel, it's a generic term for a city castle. This country also has some Romantic revival castles; these are likely to be found in rural areas.
Japan. Japanese castles, named shiro, are remarkably different in architecture from European ones, because they were built to protect the local feudal lords from local siege tactics, which, differed significally from classic medieval European sieges. Most notably, firearms found a very slow and lukewarm welcome in feudal Japan, and their introduction only stimulated castle building rather than put an end to it.
The Near East. Castles were introduced to this area by the crusaders, and, surprise-surprise, were completely based on European designs. However, citadels had been common for centuries, particularly in the Levant (which had been one of the most fought-over regions in the world basically since people could write); although most surviving Middle Eastern citadels date from the Crusader period or just before, one, the Tower of David in Jerusalem, has been a citadel since the 2nd century BCE (albeit one that kept getting destroyed and rebuilt, like the rest of the city).
India. This country's equivalents of castles are called durga in Sanskrit or qila in Hindi. These words were usually translated as "forts", because they were used as army forts by the British colonial army, but they were originally castles.
Famous, unique and unusual castles around the world
Neuschwanstein. The archetypical Romantic revival castle, it was built by the eccentric king Ludwig of Bavaria ("Mad King Louie" or the "Moon King") when Bavaria was actually part of the federal German Empire but retained its status as an "independent realm." Everyone might recognize this castle as the one shown in the Disney logo and copied in Disneyland parks as the "Sleeping Beauty Castle". This is not surprising, because Ludwig purposefully designed this castle as the fairy-tale castle of his dreams. It is a very popular tourist spot.
St.Michael's Castle, St. Petersburg. Another revival castle, this one is unique in many ways. First, despite being a revival castle, it had a genuine defensive function: it was built by a Properly Paranoid Russian emperor Paul I to serve as his highly secure residence. Second, it has a unique "four-sided" architecture: the four facades of this quadrangular castle show different architectural styles each, from neo-gothic and pseudo-medieval to generic XVIII century palace. It didn't serve it intended function, its owner was killed by conspirators, the castle's defensive moat was filled with ground and it was repurposed as an engineering school (hence its second name, Engineer's Castle). Currently it houses the Russian Museum of Art and is open for visitors.
Krak des Chevaliers, Syria. The archetypical Crusader castle in the Near East, one of the largest and the most well-preserved (well, until the late unpleasantness in Syria began). This large concentric castle belonged to the Order of Knights Hospitaller, a famous order of knights who guarded the safety of pilgrims in the Holy Land and eventually evolved into the Sovereign Order of Malta, which still exists to this day.