A keep (from the Middle English kype) is a type of fortified tower built within castles during the Middle Ages by European nobility. Scholars have debated the scope of the word keep, but usually consider it to refer to large towers in castles that were fortified residences, used as a refuge of last resort should the rest of the castle fall to an adversary. The first keeps were made of timber and formed a key part of the motte and bailey castles that emerged in Normandy and Anjou during the 10th century; the design spread to England as a result of the Norman invasion of 1066, and in turn spread into Wales during the second half of the 11th century and into Ireland in the 1170s. The Anglo-Normans and French rulers began to build stone keeps during the 10th and 11th centuries; these included Norman keeps, with a square or rectangular design, and circular shell keeps. Stone keeps carried considerable political as well as military importance and could take up to a decade to build.

During the 12th century new designs began to be introduced - in France, quatrefoil-shaped keeps were introduced, while in England polygonal towers were built. By the end of the century, French and English keep designs began to diverge: Philip II of France built a sequence of circular keeps as part of his bid to stamp his royal authority on his new territories, while in England castles were built that abandoned the use of keeps altogether. In Spain, keeps were increasingly incorporated into both Christian and Islamic castles, although in Germany the use of tall towers called bergfried, rather than keeps in the western fashion, were preferred. In the second half of the 14th century there was a resurgence in the building of keeps. In France, the keep at Vincennes began a fashion for tall, heavily machicolated designs, a trend adopted in Spain most prominently through the Valladolid school of Spanish castle design. Meanwhile, in England tower keeps became popular amongst the most wealthy nobles: these large keeps, each uniquely designed, formed part of the grandest castles built during the period.