The Castle-class minesweeper was a highly seaworthy naval trawler adapted for patrol, anti-submarine warfare and minesweeping duties and built to Admiralty specifications. Altogether 197 were built in the United Kingdom between 1916 and 1919, with others built in Canada, India and later New Zealand. Many saw service in the Second World War.

The Castle class originated with the commercial trawler Raglan Castle of 1915,^[1][3][4][5] taken up for Royal Navy service. The design was adapted by Smith's Dock Company Ltd to Admiralty requirements for building in quantity.

During World War I, 145 were built in the United Kingdom for the Admiralty. The names of the vessels were derived from the official crew rosters of ships at the Battle of Trafalgar.^[5] Non-standard ships included in the class varied between 236 GRT and 276 GRT, with dimensions varying between 35.8 and 38.3 m (117 and 126 ft) length and 6.71 and 7.17 m (22.0 and 23.5 ft) beam. The first standard vessel, Nathaniel Cole, was delivered in May 1917. After the war 52 further ships on order were completed as fishing vessels and many of the minesweepers were converted for commercial use. 20 ships were also cancelled, but many were completed by the shipbuilders for commercial owners.

The Admiralty also ordered 60 Castle-class trawlers from Canadian shipyards, the TR series, which were loaned to the Royal Canadian Navy for seaward defence of the east coast of Canada. Some entered service with the United States Navy. Nine ships were also built in India with teak planking on steel frames.^[1]

Many Castle-class trawlers were among the civilian trawlers requisitioned by the Royal Navy for use in World War II. The majority were employed as minesweepers, with others serving as auxiliary patrol vessels, boom defense vessels, danlayers, Essos (fuel carriers), torpedo recovery vessels and water carriers. Many had multiple roles over the course of the war. After the end of the war, most of those that had survived were either sold or returned to their owners.^[6]