Caerphilly Castle is a medieval fortification in Caerphilly in South Wales. The castle was constructed by Gilbert de Clare in the 13th century as part of his campaign to maintain control of Glamorgan, and saw extensive fighting between Gilbert, his descendants, and the native Welsh rulers. Surrounded by extensive artificial lakes – considered by historian Allen Brown to be "the most elaborate water defences in all Britain" – it occupies around 30 acres (12 ha) and is the largest castle in Wales and the second-largest castle in the United Kingdom after Windsor Castle. It is famous for having introduced concentric castle defences to Britain and for its large gatehouses. Gilbert began work on the castle in 1268 following his occupation of the north of Glamorgan, with the majority of the construction occurring over the next three years at a considerable cost. The project was opposed by Gilbert's Welsh rival Llywelyn ap Gruffudd, leading to the site being burnt in 1270 and taken over by royal officials in 1271. Despite these interruptions, Gilbert successfully completed the castle and took control of the region. The core of Caerphilly Castle, including the castle's luxurious accommodation, was built on what became a central island, surrounding by several artificial lakes, a design Gilbert probably derived from that at Kenilworth. The dams for these lakes were further fortified, and an island to the west provided additional protection. The concentric rings of walls inspired Edward I's castles in North Wales, and proved what historian Norman Pounds has termed "a turning point in the history of the castle in Britain".

A castle is a type of fortified structure built during the Middle Ages predominantly by the nobility or royalty and by military orders. Scholars usually consider a castle to be the private fortified residence of a lord or noble. This is distinct from a mansion, palace and villa, whose main purpose was exclusively for pleasance and are not primarily fortresses but may be fortified. Use of the term has varied over time and, sometimes, has also been applied to structures such as hill forts and 19th- and 20th-century homes built to resemble castles. Over the Middle Ages, when genuine castles were built, they took on a great many forms with many different features, although some, such as curtain walls, arrowslits, and portcullises, were commonplace.