One of the most important features in a castle was its walls. Whether made of wood, stone or brick, they provided a barrier to enemy attackers. They typically included wall walks, which were used by the defenders to resist attempts to scale the walls or to shoot missiles at the besiegers.

While movie-magic often focuses on the drawbridge, the most important thing for a medieval castle to have for true survival was a reliable, internal water source, typically a deep stone-lined well. Without water, a castle’s impressive defenses—like its 6-foot-thick curtain walls—were useless, as a besieging army could simply wait for the defenders to die of thirst within a few days. Architecturally, the most critical defensive feature was the gatehouse. Because the entrance was the weakest point, it was transformed into a formidable "killing zone" equipped with multiple portcullises, heavy oak doors, and murder holes in the ceiling through which boiling water or projectiles could be dropped. Later "concentric" designs added layers of defense, but the core requirement remained a secure interior well; a castle could survive a siege for months with a well-stocked granary and a constant supply of water, whereas the grandest towers in the world would fall in a week without it.