Fort Tregantle was built as a defensive structure to serve during a possible invasion by French forces in the 1850s. There was a general sense of alarm triggered by France's naval build-up, and the prime minister of the day, Lord Palmerston, commissioned the construction of around 30 forts, earthworks and gun batteries around Plymouth, which was seen as a prime target for the French.

Building started in 1858 and took around six years. By the time they were completed, the political situation and advancing technology rendered them obsolete. The structures became known as 'Palmerston's Follies'. However, some of them became important training grounds for the armed forces. The Monmouthshire Regiment went to Fort Tregantle annually to undertake excercises that could not be attempted on their own training ground.