Fort Amherst was initially built as a redoubt to secure the southern end of the Great Lines. At this point it had three adjacent batteries, Cornwallis Battery, Belviderre Battery, and Prince William Battery. Construction began in 1756, with major works carried out between 1806 and 1811.
The later improvements to Fort Amherst included extension of the underground tunnels and three Haxo Casemates below Lower Cornwallis Battery. The Fort was utilised during the Victorian period but new fortifications were built. The existing emplacement were still utilised, with some new gun positions built for more modern guns (68-pdr RMLs).