There are no studies devoted solely to blockhouses, which have been considered only in general works on castle studies, of which there are many. The subject of artillery fortifications has, however, received an impetus by the founding of the Fortress Study Group and the annual publication of their journal Fort; the subject also comes within the remit of the Society for Post-Medieval Archaeology . The latest volume of the History of the King's Works deals with the 16th century examples of this class of monument; earlier examples are covered by J R Kenyon (1981) in Early Artillery Fortification in England and Wales. A full catalogue of English castles is by Cathcart King (1983) in Castellarium Anglicum, whilst his synthetic work of 1988 The Castles in England and Wales has a useful chapter (no. 14) on Henry VIII and coastal defence. Investigation of the physical remains of blockhouses has been largely through the study of the standing structures.
Although many castles were adapted in minor ways for the use of artillery (see Kenyon 1981) the first gun-tower or blockhouse to be built incorporating gunports seems to have been the Cow Tower in Norwich c1398 which was of three storeys the upper two of which each had six gunports. The first phase of Camber Castle, Sussex, 1512-1514, was also a tower, which after the addition of bastions became the citadel of an artillery castle. Many blockhouses thereafter had a tower but always with a bastion(s) or gun platform(s), with the whole complex having a much lower silhouette and with the main armament on the only, or lowest, level. There is a wide diversity of layout; there may be a tower with bastions or a gun platform, or a bastion only, with a great variety of shapes of all major components. Holbein's painting of Henry VIII at Dover c1550 shows a tower blockhouse in use (see Figure 1).
The principal components, not all of which may have been provided, may be a tower, and one or more bastions or alternatively gun platforms.
The early towers were at least two storeys high - Cow Tower was of three storeys with a total height of c17m, was circular with an internal diameter of c8m, and was strongly battered. Later towers are lower and usually square or rectangular - Sandsfoot, Dorset, was of two storeys and 15m by 9m internally, whilst Netley, Hampshire, of similar size in plan, was single storey but with a parapet at roof level. Key features in the tower would be embrasures, pistol loops, gunports, ammunition alcoves, and smoke vents; there might be internal rooms for garrison use, a fireplace, oven, and latrine. There might, rarely, be a guardroom, as at King Charles' Castle, Isles of Scilly.
The bastions, when built without a tower, were commonly D shaped (Gravesend, Kent), but with a tower were varied in shape; Sandsfoot was a regular polygon, and Sheerness, Kent, was irregular, whilst those at Hull, Yorkshire, were of pointed-segmental form. Bastions were of much the same size, with axes of the order of 10-12m, and were not always roofed, enabling gunsmoke to disperse. Key features were embrasures, which might have ammunition alcoves and slots to carry gun rests, and sometimes there was a wall-walk and battlements in the wall above, or at roof level. Where the bastion was roofed, smoke vents would be formed in the roof of the embrasures. Fireplaces (Little Dennis, Cornwall) and ovens (St Mawes, Cornwall) were occasionally provided.
Gun platforms were always provided with a tower and were varied in shape and size; Sandown, Isle of Wight, had a rectangular platform 40m wide and 30m deep, with a projecting semicircle on the seaward side, Brownsea, Dorset, had an irregular pentagon 27m wide on the seaward side, whilst Netley had two, one each side of the tower, each being rectangular about 12m by 7m. Key features would be a parapet which would be rounded on the outside top edge, and have crenellations for guns; the crenellations might have slots for gun rests and sockets for shutters.
No classification for blockhouses has been published and the following provisional scheme is proposed.
Type I. Tower only (eg. Camber, Sussex) II. Bastion only (eg. Little Dennis, Cornwall) III Tower with bastion (eg. King Charles' Castle, Tresco) IV Tower with Platform (eg. Cromwell's Castle, Tresco) Relationship of bastion or platform to tower A. Concentric (eg. Sheerness, Kent) B. Linear (eg. Netley, Hampshire) C. In front of tower (eg. Sandsfoot, Dorset) D. Offset (eg. Tilbury, Essex) Shapes a. Round tower (eg. Cow Tower, Norfolk) b. Square or rectangular tower (eg. Netley, Hampshire) c. Semi-circular bastion(s) (eg. St. Mawes) d. Polygonal bastion (eg. Sandsfoot, Dorset e. Segmental pointed bastion (eg. Hull, Yorkshire) f. Rectangular platform (eg. Brownsea, Essex) g. Polygonal platform (eg. King Charles' Castle, Tresco) h. Irregular platform (eg. Tilbury, Essex)
Classifications will be given in the above order, ie. IIICbf and so on.
Figure 1 shows a selection of typical plans.
Blockhouses were owned by, and held on behalf of the king, although allegiance could, and did, change, during the Civil War. They were powerful defensive structures designed to protect the coast, or maritime or river approaches, or to give outlying support to a parent castle. Accommodation, if provided, was for the garrison only.
The defensive function of blockhouses, although remaining to some degree depending on their situation, was diminished by the continued development of artillery and in some cases by changes in the coastline. There was no useful alternative use for blockhouses and most were abandoned.