A blockhouse is a small strongly built defensive structure built specifically to house guns and to protect the gunners and ammunition from attack. They are usually built of stone (more rarely brick) and were typically sited to command a river, harbour entrance, or anchorage, or as an outlying work to provide enfilading fire or protection to other defensive works. Accommodation within the blockhouse, if provided, was only for short-term use by the gunners or garrison. Although there was great variation in their design the main components are fairly standard, often being a tower dominating a bastion or gun platform. There may be a dry ditch or moat, or earthen defences on the landward side. They will often be recognised as (relatively) low upstanding structures.
Sites that may be confused with this class are artillery castles which are considered as a separate class of monument. Both classes filled the same defensive function, were often similar architecturally, with many being built in association with one or more of the other class; a blockhouse will be recognised as a single free-standing structure with only two or three main elements whereas an artillery castle will be a complex of several interdependent structures. In documentary sources of the 16th century the term "bulwark" is often used indiscriminately for blockhouses of stone and for earthen forts; the latter are excluded from this description and are considered as a separate class of monument.
Specifically excluded from this description are blockhouses or gun-towers forming an integral part of the enceinte of castles; the 19th century forts, built for much larger guns and usually sited further inland on high ground; the 19th century maritime forts built in the Solent; and the Martello gun-towers of the Napoleonic wars.
Blockhouses were designed solely to protect a particular feature or area by the use of artillery against an attacker similarly armed. They had accommodation only for the short-term use of the gunners or garrison. The first known example is the Cow Tower, Norwich, built in 1398, and a late example is Cromwell's Castle, Isles of Scilly, built in 1651; most were built in the first half of the 16th century.