A keep 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, Portugal, south Italy and Sicily. As a result of the Norman invasion of 1066, use 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, including 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 a decade or more to build.

The Château de Langeais is a 15th-century Flamboyant Gothic castle in Indre-et-Loire, France, built on a promontory created by the small valley of the Roumer River at the opening to the Loire Valley. Founded in 992 by Fulk Nerra, Count of Anjou, the castle was soon attacked by Odo I, Count of Blois. After the unsuccessful attack, the now-ruined stone keep was built; it is one of the earliest datable stone examples of a keep. Between 994 and 996, the castle was besieged unsuccessfully twice more. During the conflict between the counts of Anjou and Blois, the castle changed hands several times, and in 1038 Fulk captured the castle again.