Portland Harbour was conceived in the first half of the 19th century as a harbour of refuge though it quickly became of strategic value to the Royal Navy. The first plans encompassed two breakwater arms, one jutting out eastward from Portland and the other northwards stretching half the way across Portland Roads towards the Nothe peninsular on the Weymouth shore. Between these two arms was to be the South Ship Channel protected from those visitors with evil intent by a small fort at the end of the shoreward arm, called the Inner Pierhead Battery. This battery was armed with 68 pounder cannons. Stretching across the South Ship Channel was a removable boom, a string of raft-like objects connected with stout wire, and nets stretching to the sea bed. The boom was opened and closed by machinery situated in D Head, the circular end of the Outer Arm, with the wire cable running through rollers in the Inner Pier Head Battery.

The open area to the north of the Outer Arm was covered by the guns mounted in the Breakwater Fort (at the north end of that arm) and a series of armed dolphins with nets slung between and stretching to the Weymouth shore at Bincleaves.

At the start of WWI the entrances to the harbour were thus defended by guns mounted on the Inner Pierhead Battery, the Breakwater Fort and A, B. and C Heads with a mixture of 12 inch, 12 and 6 pounder guns, and a Maxim machine gun - the latter on the Inner Pierhead Battery. Each entrance could be closed by net, operated on a push-pull basis, and there were also torpedoes mounted to cover the East and North Ship Channels. The South Ship Channel was thought to be vulnerable to torpedo attack and so in 1914 an old battleship, the first HMS Hood, was sunk across the entrance.

By 1940 the breakwaters bristled with additional equipment, searchlights and gun control towers, spigot mortars, and two old Fordson tractor engines to operate winches which opened and closed light anti-swimmer nets. The main 1900 booms were replaced by equipment working on a curtain principle operated from new machine houses built on the breakwaters.