The Round Tower is part of the first permanent defensive works to be built in Portsmouth. Conceived as one of two towers, one on each side of the harbour entrance with a chain boom stretched between them, it was likely to have been erected during the first part of the 15th Century. The tower itself has been subject to many structural alterations and was probably largely reconstructed during the reign of Elizabeth I. Another phase of building work was undertaken during the Napoleonic era when the building appears to have taken on a stronger defensive role.

In the 1960s a bronze relief map of the harbour mouth, the eastern Solent and the Isle of Wight was installed on the circular top of one of the gun emplacements. Regrettably, during some repair work on the tower over a decade ago the plaque went missing.

In recent years there has been significant progress in raising awareness of the value of the information plaque and obtaining support from local Councillors, residents and members of The Friends of Old Portsmouth Association (FOOPA) to try and get the plaque replaced. In the past year officers from Portsmouth City Council’s Culture Service have been supporting the Friends of Old Portsmouth Association in a project to get the plaque replaced. The project has been funded by the Friends of Old Portsmouth Association, Cllr Worley’s Member’s Neighbourhood Initiative fund and PCC Arts Service.

The new bronze plaque is a low relief toposcope which was installed and unveiled in July. It has modelled relief detail of the view from the tower and features significant landmarks and sites of interest such as the site of the wreck of the Mary Rose, the Spinnaker Tower, Spitbank Fort and the spire of All Saint's Church in Ryde.