This ship was laid down as Almirante Cochrane, sister to Almirante Latorre, Dreadnought battleships for Chile, but work was suspended at the outbreak of WWI. Latorre was more advanced, so was completed as HMS Canada, and in 1917 Cochrane was purchased incomplete for conversion to an aircraft carrier, and towed to Portsmouth Royal Dockyard.
The general concensus had moved from seaplane or 'ramp and hangar' carriers to a full end-to-end through flightdeck. the major innovation howerver, with experience from Argus was the revolutionary island - the funnels, masts, AA armament, bridgework and uperworks were moved to the starboard of the flightdeck to form an island, leaving the flight deck area free for planes. ( The reason for the right hand island was that the clockwise whirling mass of early rotary engines, and later very powerful radials and inlines, caused a notable swing to port on takeoff. The island position prevented any swing damaging the bridge, although the poor pilot would undoubtedly end up in the drink! ) This feature was copied by all other navies and most other aircraft carriers ever since, and is still the norm.
However, a heavily built battleship hull was not the ideal vessel for a carrier, and despite having the highest hangar in the fleet, had the lowest ever aircraft : displacement ratio of any RN carrier ever. A lot of weight and space was taken up by unnecesary strengthening, and the space was very hard to utilise effectively. Despite this she was the most heavily armed (in surface terms) and the stotest built carrier the RN ever had, and she wasvery well accepted - being years ahead of the rest of the world at the time.
Although not intended for surface action, and to work as a part of the fleet ( scouting and providing fighter and strike cover ), the armament was comprised of nine single 6" guns, heavier than any 6" cruisers of the time for beating off attacks by cruisers, destroyers or merchant cruisers. Six positions for 4" AA guns were provided around the bridge, although only five guns were ever fitted, and the upperworks carried the director positions.
Just pre-war, a HA/DCT was added on the fore-top, an octuple pom-pom was added between the two funnels, and four quad machine guns were added.
Later, a second pom-pom was added, the machine-guns were replaced by twelve 20mm Oerlikons along the flight deck sides, AW type 290 and AR type 285 were added, and some furnace-oil bunkers were replaced by aviation fuel storage.
Her and here sister ( later returned ) were renowned across the navy for their engine room instruments which displayed only in Spanish and metric! Eagle was also the only ever ( real ) British carrier with more than one funnel, two stout units being fitted atop the island.