Nowhere is this more vividly demonstrated than in the history of the defences of the great deep-water port of Portland, a prime example of the rapid development of Victorian military engineering. In an incredibly short time the defences developed from batteries of low velocity, simple, short ranged, muzzle loading, Trafalgar type cannons to emplacements of powerful, high velocity, breech loading guns that were to see service right up to World War 2.

The Breakwater Fort was but one such battery, and others were built at the Nothe at Weymouth, on the cliffs of Portland at East Wear, on the end of the Inner Pier of the Breakwaters and, in the 20th century, at Blacknoor and Upton. A great citadel was carved from the stone at the Verne on the heights of Portland and, placed just to the citadel's south, a High Angle Battery was situated to lob shells from on high at the vulnerable decks of attacking warships.

As well as these powerful forts each entrance to the harbour had its own defence with one or two gun positions, searchlights, torpedoes and a system of removable net to deter submarines and swimmers.