In medieval architecture, a keep (also known as a donjon) is specifically the central, strongest tower or primary fortified residence within a castle complex. While a "castle" refers to the entire fortified enclosure—including the outer walls, towers, gatehouse, and baileys—the keep was designed as the final refuge of last resort if the outer defenses were breached. Keeps were characterized by their massive thickness, often featuring living quarters, a chapel, and storage for a long siege. Famous examples like the White Tower at the Tower of London illustrate how a keep can be a standalone fortress in its own right. Essentially, every keep is part of a castle (or was meant to be), but not every building in a castle is a keep; only the most heavily fortified, multi-story central structure earns that title.