A barbed wire fence surrounded the Fortress. The extensive underground work was started in November, 1942. Unlike similar emplacements overseas, here the guns were to be serviced by a completely underground system of tunnels and magazines. This entailed building three magazines with shafts for ammunition hoists to the guns, an engine room with exhaust chamber and main air ventilating shaft, an oil store, a Fortress and Battery plotting room, three gun stores, a command post, and stairways and landings to three gun pits, as well as the interconnecting tunnels. In the latter stages a lot of trouble was experienced with water seepage through the concrete linings of the tunnels but this was fixed by chasing the joints and leading the seepage to the underground drainage system. The DSIR were also called in to modify the air conditioning system because of problems with condensation.
Two massive 185 horsepower Ruston and Hornsby diesel generators provided the power to manoeuvre the guns and a smaller auxiliary generator gave power for lighting and air conditioning. The main engines produced 120 kw DC power and the smaller 24 kw DC.
Two 9.2 inch guns(23.3cm) were installed by the Army in l944. Parts of the guns arrived by ship from the United Kingdom and were transported from the Wellington docks up Glenmore Street to Karori, and up the steep and narrow unsealed Wrights Hill Road to the summit. The guns, complete with turret, each weighed 135 tons. The gun barrels alone weighed 28 tons, and had to be towed up the hill behind a D8 tractor. The order for the third gun was cancelled after an improvement in the Pacific War situation and gun pit number three was used to store the gantry runways, the gantry and other gear.
The 9.2 inch (23.3 cm) guns could fire a 3801b (172 kg) shell up to 18 miles (29 km) to the entrance of Wellington harbour into Cook Strait and across towards Tory Channel in the Marlborough Sounds. Each shell was about three feet (1 metre) long and was propelled by two half-charges of 62 lbs (28 kg) each of cordite. The guns were water cooled and ran on massive roller bearings. As with all NZ coastal guns, they were never fired in anger. After the war had ended the Army was keen to test fire or “proof” Number One gun after the War had ended. The date was June 28th l946, and three rounds were fired into Cook Strait. The Army reported that the "proofing" was most satisfactory.
The Fortress was used for training purposes in the 1950’s but its operational days had come to an end, and regular maintenance was carried out by teams from Fort Dorset. In the early 1960s a Government decision was made to cut the guns up for scrap and sell them. A Sydney firm landed the contract and it also removed the guns from Palmer Head, Fort Dorset and Fort Ballance, as well as from Auckland and Christchurch. The gun emplacement pits were later filled in with rubble, and other equipment was removed from the Fortress. The PWD put the whole project at £250,000, which would exceed about $20 million (NZD) in today’s terms.