The design was adapted by Smith's Dock Company Ltd to Admiralty requirements for building in quantity.

The names of the vessels were derived from the official crew rosters of ships at the Battle of Trafalgar.

After the war 52 further ships on order were completed as fishing vessels and many of the minesweepers were converted for commercial use.

[6] Early in World War II, ten Canadian-built trawlers that had been sold into commercial service after World War I with a number of European countries were captured by the Germans when they overran France, Belgium and Norway and put in Kriegsmarine service.

The Castle-class design was chosen over more modern alternatives because it was simple enough to be built with the country's limited ship construction facilities.