A stone keep is a massive, fortified central tower that served as the primary defensive stronghold and residence within a medieval castle. Emerging in the late 11th century (most notably during the Norman conquest of England), stone keeps were developed to replace the more vulnerable wooden towers of earlier motte-and-bailey castles. Built with thick stone walls—often ranging from 10 to 15 feet in thickness—these structures were designed to be virtually fireproof and resistant to siege engines. The most famous example is the White Tower at the Tower of London. A stone keep typically featured a "raised entrance" on the first or second floor, accessible only by a removable wooden staircase or a heavily guarded "forebuilding" to prevent direct attacks on the ground floor. Inside, the keep was a self-contained world, containing the Great Hall for feasts, private solar chambers for the lord's family, a chapel, kitchens, and deep storage cellars for food and water. While their primary function was military defense, their sheer verticality and imposing masonry also served as a powerful visual symbol of the lord's authority and permanent control over the surrounding landscape.