Three of the four main periods of artillery fortification in England, when construction was undertaken on a national scale, are represented in the Thames: the castles and bulwarks of Henry VIII, the bastioned forts of the late seventeenth century, and the Royal Commission forts of the 1860's. The exception is the rather special case of the Martello Tower system, but its place here is taken by the steady improvements carried out at the end of the eighteenth century and in the 1840's and 1850's. Shornemead, Cliffe Creek, and Slough Forts are ruined and derelict, Coalhouse Fort is used as a store, but Tilbury, now in the guardianship of the Ministry of Works, can show the progress of fortification over the course of 250 years. That they never saw the action for which they were designed is irrelevant, for, as Sidney Herbert said in 1860, ‘the object of the fortifications is not so much to resist as to deter attack’.
The subsequent coastal batteries of the last two wars, the hastily constructed pill-boxes, and even the ‘flak’ towers off the east coast are simply gun positions. They are neither self-contained nor self-defensible fortresses. By these criteria the forts erected as a result of the 1860 Royal Commission can be regarded as the last legitimate successors of the prehistoric hill fort and the medieval castle.