Former Gymnasium at St Vincent Centre

This mid-to-late 19th-century building, originally a gymnasium within the former Royal Marine Barracks, is located off Gosport Mill Lane. It comprises two distinct construction phases. The building is constructed of red brick in English bond, with a later addition in Flemish bond. Ashlar dressings are present throughout, and the roof is covered in Welsh slate, with replacement felting to one section.

The main building, with a gabled elevation, is arranged with three bays by four bays, truncated at the rear by a one-bay-deep, four-bay-wide addition. The gable end features a chamfered plinth, pilasters near each end, and corners offset at eaves level. The central bay projects slightly and contains a full-width entrance with late 20th-century boarded doors, cyma-moulded stone imposts, and a concreted lintel. Above the entrance is a lunette with a gauged brick arch, a tripartite keystone, and a small-pane tripartite window. The gable has moulded stone coping. The right return features alternate narrower bays with blind recesses having chamfered bases and stepped tops; the end recesses are narrower, the left with a concrete step up to an iron-hinged metal door and concrete lintel. Windows on the left return are segmental brick arched, with raised brick keystones and paired 2-light 4-pane casements. Three further windows, wider than the first two, feature three similar casements, stone imposts, and evidence of former shutter holders above and below. Most windows have metal frames. A glazed clerestory runs along the ridge. The left return is similar, but with only three recesses and formerly two wide window openings, now partly bricked up and each containing three small-pane wooden windows, stone imposts, a wooden shutter holder above, and brackets from a former lower shutter holder.

The rear addition has a round-arched recess in each bay of the end and return elevations, containing similarly-arched openings, stone sills, gauged brighter-red brick arches, and an impost band. On the right-hand end elevation, there are four stone steps leading to a former doorway, now bricked up; other recesses contain boarded-up windows, with the second having a late 20th-century door leading up steps. A coped parapet tops the structure, and there are stacks near the ends of the ridge.

The interior features a cambered board floor, with some concrete insets, a kick board along the side walls, round-edged sills, steel lintels to windows, diagonally-boarded roofing, braced steel trusses, and hinged louvres at the base of the clerestory. Three doorways lead into the addition, set approximately one metre above the floor and with stone sills. A small former strong room is located in the front right corner. It is believed the building may have been used as a gun training deck due to its cambered floor, although it is dissimilar to the known gun training deck at Plymouth.