The structure that survives
The ring is built onto the rock of Battery Hill. Historic Environment Scotland describes bull-faced squared and snecked granite on the exterior and hammer-dressed random rubble within. The east entrance is round-arched and set in a raised crenellated section.
The 94 pointed openings are commonly counted as 44 in the lower tier and 50 in the upper tier. The National Record of the Historic Environment gives the circumference as approximately 200 metres.
Roman arena, Gothic language
Calling the tower a copy of the Colosseum is convenient but imprecise. Its circular arena-like conception recalls Roman precedents; its pointed arches, battlements and proposed ‘old English’ central tower belong to a Gothic vocabulary. The mixture is deliberate and distinctive.
The unbuilt centre
An 1895 newspaper description reported plans for a tower about 95 feet high on a 20-foot platform, producing a total height of roughly 120 feet. It was to contain a zig-zag stair and public viewing platform. A choir chapel, museum and art gallery were also contemplated, connected by groined arcades.
What remains uncertain
Two surviving sketches associated with the project are held by Oban War & Peace Museum. The museum has attributed them to McCaig, but academic analysis notes that their authorship is not conclusively established and suggests they may be by different hands.