Fort Tregantle was originally designed before the Royal Commission Report of 1860, but the plans were accepted with modifications. This fort was primarily for land defence, but its location also gave it a limited coastal defence role. Tregantle Fort was built between 1859 and 1866 to mount:
- Twenty two 7-inch RBL guns
- Thirteen 64-pdr RML guns
- Twenty four 32-pdr SB for close defence
- Five 8-inch Rifled Howitzers
The fort was never fully armed and in 1893 had only the following armament:
- Five 7-inch RBL guns
- Nineteen 64-pdr RML guns
- Ten 32-pdr SB for close defence
As with many other Forts of this period in Portsmouth, there was a keep and enceinte with four capniers.
