A keep is a strong central tower which is used as a dungeon or a fortress. Often, the keep is the most defended area of a castle, and as such may form the main habitation area, or contain important stores such as the armoury, food, and the main water well, which would ensure survival during a siege.
An earlier word for a keep, still used for some medieval monuments, especially in France, is donjon; a derivative word is dungeon. In Germany, this type of structure commonly is referred to as a bergfried.
Often early keeps were just square towers with very thick walls, scarcely more than a residential hall, such as Château de Langeais. This structure later developed into the more recognizable rectangular residential keep by the eleventh century.
The form and function of the keep changed with time and varied depending upon the region where it was built. As the keep was a defensive structure, the shaping trend changed to adapt to the developments in weapon technology. For example, the round or cylindrical keep was first introduced as a defense against the battering ram. A battering ram could cause severe damage to a side of a square tower, but merely would glance off the side of a rounded one. Also, a round tower is much harder to undermine successfully than a square one. Rounded towers also have the advantage of less "dead ground", or, areas not visible from the tower summit.
By the early twelfth century, cylindrical keeps had become popular and they remained prominent in France until the end of the medieval period (e.g. Montlhéry, Rouen). Variations on the rounded type began to appear at the same time. These included towers with triangular, prow-like projections (such as Château-Gaillard), polygonal keeps such as at Orford, or "multi-lobed" keeps such as Clifford's Tower.
From the early thirteenth century onward many castles were designed without traditional keeps, instead the preferred plans for defensive structures were concentrated in the walls and towers of the enceinte, or in a gatehouse. Early examples may be seen at Château de Boulogne-sur-Mer, in Boulogne-sur-Mer and Bolingbroke Castle in Bolingbroke, Lincolnshire, and later, at The Bastille. In some castles the gatehouse took over the functions of the keep, serving as refuge, residence, and command post, such as at Harlech Castle.
As nobles became more interested in grand halls and comfortable living quarters, the keep lost its domestic role. Although keeps continued to be used and built, there is evidence that many had a reduced role, demonstrated by the lack of residential amenities in the tower plans, such as latrines and chimneys.
In Western Europe, however, the defensible residential keep experienced a resurgence before the end of the medieval period, as towers were built to house nobles and their retinues securely, but at a very high level of comfort and luxury (e.g. Raglan Castle, Ashby de la Zouch Castle, Château de Vincennes, and Château de Largoët). This luxurious type was particularly popular in late medieval Scotland up until the 1600s (e.g. Craigievar Castle). Another word for this type of keep is the tower house.
Shown to the right is the tallest keep remaining in existence, the donjon of Château de Vincennes, which is located in a suburb of contemporary Paris. Previously this distinction was held by the donjon of Coucy in Picardy.