An artillery castle is a powerful defensive structure built specifically to house heavy guns, usually forming concentric defences with the guns in multiple tiers . The artillery castle walls were built of stone, sometimes with earthen outworks. They were usually sited to protect a harbour entrance, anchorage, or similar feature. Accommodation within the castle, if provided at all, was usually limited to that needed for the garrison alone. Although there was great variation in the design of this class of castle the main components are fairly standard: usually a central citadel or tower, bastions set round the citadel, a gatehouse, ward, and sometimes an encircling ditch or earthen curtain and bastions.

Sites that may be confused with this class are blockhouses which are considered as a separate class. Both classes filled the same defensive function, were often similar architecturally, with many being built in association with an example of the other class; a blockhouse will be recognised as a single free-standing building whereas an artillery castle can be distinguished by its complex of several interdependent structures.

Excluded from this description are the 19th century forts, built for much larger guns and usually sited further inland and on higher ground than the artillery castles; 20th century gun emplacements of which there is a great variety; and Martello gun- towers of the Napoleonic wars. These are all considered as separate classes.

Artillery castles were designed solely to defend a particular feature or area by the use of heavy guns, against an attacker similarly armed; they seldom had accommodation for more than the gunners and garrison. Dartmouth Castle, Devonshire, was the first to be designed for heavy guns in 1481; most were built between 1539 and 1545. The last was at Upnor, Kent, built 1559-61.