An Admiralty semaphore station was built at Pewley Hill in 1822 forming part of the London-Portsmouth semaphore line. The building remains, with a cupola on top which was added after the semaphore went out of use. In 1866 Guildford Corporation built a new reservoir at the top of Pewley Hill. Pewley Hill Fort (or Mobilization Centre) was built in around 1890 as part of the London Defence Positions scheme, running from Guildford along the North Downs to Fort Halstead, up the Darent Valley to Dartford, and north of the Thames to North Weald. The scheme was abandoned in 1906.