A gatehouse is a type of fortified gateway, an entry control point building, enclosing or accompanying a gateway for a town, religious house, castle, manor house, or other fortification building of importance.

A gatehouse was the most heavily fortified and strategically critical part of a medieval castle, serving as the primary entrance and the first line of defense against invaders. In the "Gold Standard" of castle design, the gatehouse was a complex "death trap" featuring multiple layers of security, including a drawbridge, a heavy wooden door, and one or more portcullises (iron-shod wooden grilles). Inside the ceiling of the gatehouse passage were "murder holes" through which defenders could drop stones, boiling liquids, or arrows onto attackers trapped below. Flanking the entrance were usually two massive towers that allowed archers to provide a "crossfire" of defense. A grounded historical detail is that as siege technology improved, gatehouses became massive, independent fortresses in their own right, sometimes containing the castle's prison or the constable's living quarters. In 2026, visiting a well-preserved gatehouse (like the one at Warwick Castle or Caernarfon) offers a supportive and visceral look at how medieval engineers balanced the need for a functional doorway with the absolute necessity of military "impenetrability."