A motte-and-bailey castle was made up of two structures, a motte, a type of mound–often artificial–topped with a wooden or stone structure known as a keep; and at least one bailey, a fortified enclosure built next to the motte. The term "motte and bailey" is a relatively modern one, and is not medieval in origin. The word "motte" is the French version of the Latin mota, and in France the word motte was initially an early word for a turf; it then became used to refer to a turf bank, and by the 12th century was used to refer to the castle design itself. The word "bailey" comes from the Norman-French baille, or basse-cour, referring to a low yard. In medieval sources, the Latin term castellum was used to describe the bailey complex within these castles.
One contemporary account of these structures comes from Jean de Colmieu around 1130, describing the Calais region in northern France. De Colmieu described how the nobles would build "a mound of earth as high as they can and dig a ditch about it as wide and deep as possible. The space on top of the mound is enclosed by a palisade of very strong hewn logs, strengthened at intervals by as many towers as their means can provide. Inside the enclosure is a citadel, or keep, which commands the whole circuit of the defences. The entrance to the fortress is by means of a bridge, which, rising from the outer side of the moat and supported on posts as it ascends, reaches to the top of the mound." At Durham Castle, contemporaries described how the motte-and-bailey superstructure arose from the "tumulus of rising earth" with a keep rising "into thin air, strong within and without" with a "stalwart house...glittering with beauty in every part".
Mottes were made out of earth and flattened on top, and it can be very hard to determine whether a mound is artificial or natural without excavation. Some were also built over older artificial structures, such as Bronze Age barrows. The size of mottes varied considerably, with these mounds being 3 metres to 30 metres in height (10 feet to 100 feet), and from 30 to 90 metres (98 to 295 ft) in diameter. This minimum height of 3 metres (9.8 feet) for mottes is usually intended to exclude smaller mounds which often had non-military purposes. In England and Wales, only 7% of mottes were taller than 10 metres (33 feet) high; 24% were between 10 and 5 metres (33 and 16 ft), and 69% were less than 5 metres (16 feet) tall. A motte was protected by a ditch around it, which would typically have also been a source of the earth and soil for constructing the mound itself.
A keep and a protective wall would usually be built on top of the motte. Some walls would be large enough to have a wall-walk around them, and the outer walls of the motte and the wall-walk could be strengthened by filling in the gap between the wooden walls with earth and stones, allowing it to carry more weight; this was called a garillum. Smaller mottes could only support simple towers with room for a few soldiers, whilst larger mottes could be equipped with a much grander building. Many wooden keeps were designed with a bretasche, a square building that overhung from the upper floors of the building, enabling better defences and a more sturdy structural design. The early 12th-century chronicler Lambert of Ardres described the wooden keep on top of the motte at the castle of Ardres, where the "first storey was on the surface of the ground, where were cellars and granaries, and great boxes, tuns, casks, and other domestic utensils. In the storey above were the dwelling and common living-rooms of the residents in which were the larders, the rooms of the bakers and butlers, and the great chamber in which the lord and his wife slept...In the upper storey of the house were garret rooms...In this storey also the watchmen and the servants appointed to keep the house took their sleep". Wooden structures on mottes could be protected by skins and hides to prevent them being easily set alight during a siege.
The bailey was an enclosed courtyard overlooked by the motte and surrounded by a wooden fence called a palisade and another ditch. The bailey was often kidney-shaped to fit against a circular motte, but could be made in other shapes according to the terrain. The bailey would contain a wide number of buildings, including a hall, kitchens, a chapel, barracks, stores, stables, forges or workshops, and was the centre of the castle's economic activity. The bailey was linked to the motte either by a flying bridge stretching between the two, or, more popularly in England, by steps cut into the motte. Typically the ditch of the motte and the bailey joined, forming a figure of eight around the castle. Wherever possible, nearby streams and rivers would be dammed or diverted, creating water-filled moats, artificial lakes and other forms of water defences.