keep

Keep, in the sense of a castle's main fortified tower, appears in English in the sixteenth century. Its origin is disputed. Some etymologists derive it from the Middle English kepen (to hold, to guard), making a keep literally 'the thing that holds.' Others connect it to the Anglo-Norman kipe or the Old Norse kippa (to pull, to seize). The earlier term was donjon (from Latin dominus, lord) — the lord's tower, the strongest structure in the castle. 'Keep' gradually replaced 'donjon' in English.

The keep was designed as the last refuge. If outer walls fell, if the bailey was overrun, defenders retreated to the keep and sealed the entrance. The Tower of London's White Tower, built by William the Conqueror in the 1070s, is one of the earliest stone keeps in England. Its walls are 4.5 meters thick at the base. The message is architectural: this building will survive after everything around it is destroyed.

Keep design evolved from rectangular (Norman, 1000s–1100s) to circular (1200s) to eliminate the vulnerability of corners — besiegers could undermine a right angle more easily than a curve. The round keeps of Conisbrough and Pembroke in England reflect this military logic. By the 1400s, keeps were being merged into larger residential castle complexes and losing their distinct identity as standalone towers.

The figurative use of 'keep' — to keep something safe, to keep a promise, to keep watch — may have reinforced the architectural meaning, or may have generated it. A keep keeps you alive. The word is its own definition.