Originally laid down as a two-decker steam ship of the line of the Bulwark class, Caledonia was converted on the building stocks into an armoured frigate.

HMS Caledonia was not completed until July 1865 due to a delay in the delivery of her main armament.

Once this was installed, she was commissioned as Second-in-Command of the Mediterranean Fleet, becoming the first ever armoured flagship of the Royal Navy.

[citation needed][dubious – discuss] On 15 June 1873, Caledonia was in collision with the British ship Hogton Tower off St. Alban's Head, Dorset.

Caledonia had been serving as a Coastguard vessel at Birkenhead, Cheshire and was sailing to Portsmouth, Hampshire for a forthcoming inspection of the fleet by the Shah of Persia.