Fort Purbrook is the most easterly of the Portsdown Forts that were built to protect Portsmouth and used to have tunnels that linked it to two even more easterly redoubts. Beneath the fort itself are a series of tunnels, magazines and gun emplacements. In World War II, these tunnels were reused as accommodation and a telephone exchange. Also during WWII the fort was used as the control centre for two bombing decoy (Starfish) sites.
Above ground, there is some evidence of modifications associated with a naval radar head. The site now operates as a children’s actvity centre and both underground magazines have unusually been converted into climbing walls.