A castle is the fortified residence of a member of the medieval noble class. Though walled cities and various types of fortresses existed long before the Middle Ages, the castle thus defined is widely considered to be an institution of medieval Europe, and its design goes hand in hand with the feudal structure of medieval society.
Some of today's most famous castles, such as Neuschwanstein Castle in Bavaria, were actually built well after the Medieval period, and others, such as Camelot, have never been proven to have ever existed outside of myth and legend.
- Battlements - Also called crenels or crenellation, battlements were a series of parapets, or low defensive walls, interspersed with narrow gaps for archers to shoot through. These were usually placed at the top of the curtain wall or turret, or at the top of a tower.
- Castle - A castle, simply defined as the fortified residence of a noble lord, is often differentiated from non-residential fortifications such as military outposts and fortifications for the common defense such as city walls. Many early castles were constructed mostly of wood in the motte and bailey design, but most castles built in the late medieval period were constructed mostly of stone with towers and crenellated walls. Some castles feature a moat and may have a drawbridge or stationary bridge, but many castles do not have moats.
- Curtain wall - The curtain wall is the outer wall of the castle, which often served as not only a formidable barrier to invasion, but also as a point of defense where guards and archers could actively defend against attacks. Battlements were sometimes put in place, especially in the late Middle Ages, to provide enhanced points of defense.
- Dungeon or donjon - The donjon (modernly called keep) was the central fortified tower within the curtain wall, which acted as both noble residence and refuge of last resort in the event of an attack or siege. These often had underground chambers used for storage and as prisons, and the term dungeon came to refer to these unpleasant surroundings near the end of the medieval period.
- Keep - The term keep refers to the central fortified tower within the outer wall of the castle. This term was not actually used in the Middle Ages, rather the central tower was called donjon, but keep has been used in modern times to differentiate from dungeon which has come to refer to the underground prison of the donjon or keep.
- Moat - A moat is a ditch, often flooded with water, surrounding a castle, especially one of motte and bailey construction, to provide an additional barrier to invasion. Though modern castle stereotypes dictate that a moat must have necessitated a drawbridge, many medieval castles in fact had stationary bridges. The terms moat and motte are both derived from Old French mote meaning "mound"; the word motte (as in motte and bailey) continued to refer to the mound upon which a castle was built, while moat came to mean the surrounding ditch left by the excavation required to build up the mound. While flooded moats provided enhanced protection against siege engines and against mining under the castle wall, many dry moats are to be found.
- Motte and bailey - One of the earliest forms of medieval castles, motte and bailey castles consist of a man-made mound upon which a fortification is built, surrounded by a ditch resulting from the excavation of the mound. This ditch, or moat, may be dry or may be flooded with water from a nearby river or stream. Though the terms motte and moat are cognates, motte refers to the mound while moat refers to the surrounding ditch.