城楼（英语：Keep、法语：donjon），一种西方中世纪建筑，为城堡中央最高的塔，用来作为监狱或是保垒之用，相当于中国的城楼，或是日本的天守。城楼是城堡中防卫最严密的地方，通常位于城堡中央的位置，用来保卫食物、水井、武器库房。当敌人包围城堡时，守军可以退守于此。

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.