The greater length of the Castles gave made them better seaboats than the Flowers, which were not originally designed for ocean escort work.

Large numbers (96 in total) were ordered in late 1942 and early 1943 from shipyards in the United Kingdom and Canada, but Allied successes in the Battle of the Atlantic meant that the requirement for escorts was reduced, and many ships (including all the Canadian ones) were cancelled.

[4] Two Admiralty Three-drum water tube boilers fed steam to a vertical triple expansion engine rated at 2,750 indicated horsepower (2,050 kW) which drove a single propeller shaft.

[4] 480 tons of oil were carried, giving a range of 6,200 nautical miles (11,500 km; 7,100 mi) at 15 knots (28 km/h; 17 mph).

[12] In the Second World War she carried out convoy escort duties and anti-submarine patrols in the Western Approaches, the English Channel and the North Sea.

[19][20] On 15 November and 26–29 December, 30 Escort Group, including Launceston Castle made more attacks against suspected submarine targets.

[21] Launceston Castle continued to serve with the 30th Escort Group for the rest of the war in Europe, but in July–October 1945, was employed on air-sea rescue duties operating out of Freetown,[14] patrolling on the expected routes of flights repatriating American troops.

[27][28] After two Avro Shackleton maritime patrol aircraft of the RAF went missing on 11 January off Fastnet Rock in the Atlantic Ocean, Launceston Castle took part in search operations for the missing aircraft,[29] while on 24 January that year she took part in search operations after a Fleet Air Arm Fairey Firefly ditched off Lands End.