This class was a progressive development from the earlier Arethusa class, and their follow ons, the Caroline / Calliope / Cambrian class. The results of which were,

1. a mixed 6" and 4" main battery was replaced by a single calibre 6" battery
2. reduction geared turbines to improve efficiency

The improvements were also inspired by two cruisers ( Centaur and Concord ) building for Turkey but taken over by the RN, in which a 6" gun was placed behind the bridge, albeit limited to broadside fire, with the addition of a third magazine amidships, between boiler rooms, to improve shell supply and also heightening sub-division thus damage control.
One thing which came out of these progressive improvements was, from the Arethusa class onwards, oil firing was adopted, thus depriving ships of the protection of the coal bunkers, and the practice of using this as the main side armour belt had to be dropped, and was, from that class onwards. Remembering these cruisers were designed in a time where aircraft at sea were a pure fantasy, all armour was devoted to the main belt, and the armoured deck was non-existant, with the main belt considered deep enough to provide against plunging shellfire ( as long as the shell hit the armoured deck! ) The lack of the armoured deck was the achilles heel of the "C" / "D" / "E" class, although it was in no way a design fault.

This group closely followed the forementioned Turkish ships, although the submerged torpedoes were replaced by twin trainable tubes above water, and the 3" AA guns were moved forward and sided the fore-funnel.
Inter war, Cassandra was lost in the Gulf of Finland during operations supporting the white Russians post WW I, and despite removing the flying-off platform only fitted in Caledon, modifications were few and far between.
Early WW II improvements consisted of adding SW type 273 on a much cut down after superstructure, and adding AS type 286 at the masheads. Five 20mm AA supplanted the two 2 pound AA guns aft, and were also placed in the wings.
In 1943, Caledon was re-armed as a light AA cruiser, with an "Arethusa" type bridge being added much further aft thanthe original, also extending forward to provide a shelter deck, she was re-rigged with tripod masts, and three twin 4" AA were added, two superfiring forward and one aft, controlled by an AR type 285 director on the bridge, and two twin 40mm AA were fitted amidships, controlled by two barrage directors equipped with AR type 282, and AR type 286 was also carried on the masthead, SW type 273 being moved aft. Six twin and two single 20mm AA were also added, the twins amidships and the single in the wings, and later, three single 40mm AA were added.
Caradoc remianed unaltered, and was paid off into accomodation duties.

Although genral follow ons of the Caledon group, one major improvement improved them imeasurably; the "Q" 6" gun was removed, the bridge moved aft to where it had been, and the missing gun was repositioned superfiring to "A" gun, on the shelter deck. Not omly did this give a drier bridge, it doubled the armament that could fire forward, whilst not reducing fire on the beams. It also meant that at least one of the for'ard guns could fire in a rough sea - this class being very wet across the bows.
Other monor alterations were deleting the conning tower, and removing the flying-off platform fitted in Coventry only.
In 1935, Coventry and Curlew were taken in hand as prototypes for converting to AA vessels - both the "C" and "D" classes being obsolecent and supplanted in their role as fleet cruisers.
( Remember however, there was absolutely no vertical protection, so any planes they could not shoot down would undoubtedly sink them with only a few bombs [!] )
All armament and aft superstructure was stripped, and the bridge modified to ease AA direction, with a HA.DCT equipped aircraft spotting top fitted to the foremast, and a second HA.DCT carried aft.Next, 10 single 4" AA guns were added;
"A" gun on the shelter deck for'ard, "B" and "C" abreast the fore funnel, "P" & "Q" en-echelon abreast aft, "R" & "S" sided on the upper deck aft, "X" on the aft shelter deck, "Y" & "Z" sided on quarterdeck.
Originally, both the forward and aft shelter decks had provision for a octuple pom-pom and it's optical director. The directors were never satisfactory and never fitted, and to ease supply problems, only one pom-pom was shipped. This conversion was judged to be successful, and they were reported to be very useful AA escorts, for the fleet or mercantile convoys.
Only Curacoa was also converted, to a modified design, similar that of the earlier Caledon, with new shelter decks carrying superfiring twin 4" AA fore and aft, and one quad pom-pom carried aft. Lastly, quad 0.5" MG's were placed abreast the fore-funnel. The outbreak of war stopped Cardiff and Ceres being converted.
Coventry was further modified early war, a tripod mainmast being added to carry AW type 286 RDF, this blocking the arcs of "R" and "S" 4" guns, so these were replaced by two twin 0.5" machine guns on the quarterdeck aft.
In 1941, Curacoa was also further modified, SW type 273 being added at the base of the mainmast, AW type 286 at the mainmast head, and AR type 285 on the HA.DCT's. Two 20mm AA were added in the bridge wings, and the quad MG's were also replaced by 20mm AA. Finally, the pom-pom was provided with an AR type 282 RDF fitted director, carried on the bridge.
The limited modifications to Cardiff and Ceres comprised of a single 2pdr AA in the Ceres along with four - Ceres - and six - Cardiff - 20mm AA, and SW type 273 was also carried on the aft superstructure. Ceres later had all her torpedoes removed, carrying four extra 20mm AA as a replacement.
In 1940, Cardiff was a gunnery training cruiser, until paying off in 1945, and Ceres was paid off as an accomodation ship in 1944.

Originally, to improve the poor sea-keeping of the bows ( any sort of sea had the bows awash in the preceding classes ) a "trawler bow", a large raised bulwark for'ard, was fitted, and a planned hangar and catapult replacing the conning tower was only carried for a short time in Carlisle.
In 1939, all except Colombo were taken in hand for conversions to AA cruisers, albeit on simpler lines than the Coventry. All except superfiring "B" gun were replaced by twin 4" AA, and "B" was replaced by a quad pom-pom, with quad 0.5" MG's replacing the 3" AA abreast the fore funnel. An AA spotting top was added to the foremast, together with a HA.DCT, another being added on a suppressed after superstructure, and a tripod mainmast was stepped.
Later additions involved replacing the quad 0.5" MG AA with 20mm AA, and adding a further six single 20mm AA along the ship, and SW typw 272 or 273 ahead of the mainmast, AW type 286 on the mainmast, AR type 285 on the DCT's and AR type 282 on the pom-pom director. Later, all except the wing 20mm AA were replaced by twin 20mm AA in the Carlisle and the Capetown.
Finally, in 1942, Colombo was taken in hand for conversion, but on different lines to her sisters;
The old 6" "B" position were the others carried a pom-pom shipped the twin 4" gun which was carried in the "X" position aft of the funnel in her sisters. In turn, "X" position carried four single 20mm AA, and "Q" position carried two sided pom-poms, port and starboard of the centreline twin 4" AA. Single 20mm AA were also shipped abreast of the mainmast.
An enlarged bridge carried a HA.DCT and was also fitted as an AA spotting position, and SW tpe 273 radar was fitted much further aft than in her siters, to narrow the blind arc across the bows. AW type 286 was carried on the mainmast, AR type 285 on the HA.DCT and AR type 282 on the pom-pom directors.