Fort Purbrook was completed in 1870 and forms part of the line of forts on Portsdown Hill, along with Fort Widley, Fort Southwick and Fort Nelson. Fort Purbrook lies to the East of Fort Widley, and was originally supported by two small redoubts known as Farlington and Crookhorn, although both have know been demolished.

The fort is now in the hands of the Peter Ashley Activity Centre, and along with Fort Widley, they are used for events and outdoor activities.

Because of its position on the flank of the hill it was constructed to bring more firepower to bear on the north-eastern approaches. A central spiral staircase leads from the parade to the underground service tunnels from the caponiers and barrack. The main magazine is off a side tunnel from the central spiral stair. The passages are the only ones in the Portsmouth defences to be equipped with chicanes.

During WW1 Fort Purbrook was armed with anti-aircraft guns and thereafter was used as a siege artillery barracks. The army left the site in 1925, but in WWII it became an ambulance base, a barracks and a centre for civil defence, housing the 65th Chemical Warfare Company in 1940, whose responsibilities included preparing for possible nuclear attack. Throughout all these years, Fort Purbrook, thankfully, never faced enemy attack.

The fort now offers various activities including:

Archery, Climbing, Assault courses, Laser Tag and much more