A portcullis is a heavy castle door or gate made of metal strips that form a grid. A castle guardian might lower the portcullis to protect the people inside from an invading army. It was common during medieval times for castles to be protected by a portcullis or two.

The main "front door" or primary entrance of a castle is most accurately called the gatehouse, though several specific architectural components make up the entrance system. The most iconic "door" itself is the portcullis, a heavy, latticed grille made of wood or iron that slides vertically in grooves to block the entrance during an attack. Many castles featured a double portcullis system to trap intruders in a "killing zone" between the two gates. Below the gatehouse, there is often a drawbridge, which acts as a movable bridge over a moat. Once inside the gatehouse structure, the actual wooden doors behind the portcullis are typically referred to as great doors or folding gates. In 2026, architectural historians also highlight the sally port, which is a smaller, secondary "secret" door used by defenders to exit the castle quickly to stage a counter-attack. While "front door" works in casual conversation, in the context of medieval fortifications, the entrance is a complex defensive machine designed to withstand battering rams and fire, with the portcullis serving as the primary visual symbol of the castle's security.