The "Marshal" class monitors that precede this class were a design disaster, and it was only a few years before replacements were schemed up; the "Erebus" class.
Originally intended to take both Ney and Soult's 15" turrets, in the end, Soult was patched up and made it into the 1930's. It was Ney's 15" turret that fitted out Terror, and the spare 15" turret from the cruiser "Furious" that was fitted in Erebus. ( Furious was designed with single 18" turrets, but two 15" turrets were placed in reserve incase the 18" proved unsatisfactory.
Originally, the light armament was 8 single 4" low-angle guns and two 12pdr AA guns, but this was modified along typical wartime standards with whatever was available. Because they were almost exlusively used in the Mediterranean, it was captured Italian guns that were fitted.
Terror was fitted with six 4" AA guns along with two HA/DCTs , to replace the LA ones previously fitted, and the two 12pdr AA were now redundant so were replaced by quad 0.5" MG's. Seven 20mm Breda cannon from Italy were fitted in 1941 at Malta, along with armour strengthening.
Erebus was in the UK for refitting so was more thoroughly refitted. Armour and the 4" guns were modified to Terror standards, four quad MG's were added and two 20 barrel 7" UP launchers were fitted. DG cables were run around the hull, despite the almost invulnerability to both this weapon and torpedoes ( length : beam ratio was an incredible 1:4 ).
From later on, the AA armament between the tow vessels was standardised. a twin 20mm Breda cannon, and three 3" UP launchers were fitted, but in 1943, this was changed to three quad and one single pom-pom, one 40mm Bofors, seven single and later four twin 20mm cannons were added - veritable floating fortresses!
SW type 272, GW type 276, AW type 79B and AR type 285 were fitted, and the full load rose to 9600 tons.