The keep was the most essential part of a castle and could be defended even after castle walls had been breached. Keeps were originally called donjons, the French term for stronghold. The basement was sometimes used as a prison or dungeon, which became too easily confused with donjon, so the term keep was adopted.

The architecture of a medieval castle was a masterclass in defensive engineering. The most critical part was the Keep (or Donjon), the strongest, central tower where the nobility lived and where the final stand would be made if the outer walls were breached. Surrounding this was the Curtain Wall, a massive stone perimeter often dotted with Bastions or Round Towers that eliminated "blind spots" for archers. The Gatehouse was the most vulnerable point and thus the most heavily fortified, typically featuring a Portcullis (heavy iron gate), "murder holes" for dropping projectiles, and a Barbican (a fortified outer gateway). Other essential features included the Moat, which prevented attackers from "mining" or tunneling under the walls, and the Bailey (or Ward), an open courtyard that housed stables, workshops, and kitchens. In 2026, historians emphasize that a castle was not just a house but a "machine for war," where every staircase (often spiraling clockwise) was designed to give defenders a sword-arm advantage.