They were designed to be built in large numbers in small shipyards without propulsion and other components (mainly gearboxes, but also guns) then in short supply so not compete with other warships for construction.

By the time the Flowers began entering service in late-1940, due to their long range they were required to undertake missions beyond coastal waters.

The Royal Navy recognized the limitations of the Flower and began designing an open-ocean escort in November 1940,[5] which became the River-class frigate.

[9] The Castle resembled later Flowers with an extended forecastle and mast behind the bridge, but was 135 tons heavier and 47 feet longer.

The Admiralty Experiment Works at Haslar developed an improved hull form which, in combination with the increased length, made the Castle at least half a knot faster than the Flower despite using the same engine.

The Castle was criticized for being barely fast enough to fight German Type VII submarines and difficult to handle at low speed.

Fifteen ships ordered for the Royal Navy from UK shipyards as part of the 1943 Programme were all cancelled on 31 October 1943: The following ships were ordered on 15 March 1943 for the Royal Navy from Canadian shipyards for completion between May 1944 and June 1945, but were all cancelled in December 1943: The final third of the film The Cruel Sea is set on the Castle-class corvette Saltash Castle (portrayed by Portchester Castle).