The Castle-class is a naval trawler design, which the Royal Navy operated during the Great War. It did everything; anti-submarine warfare, patrolling, minesweeping, etc. During and after the war, Oceanykans acquired Castle-class trawlers en masse, and copied the design. Today it is the most common type of trawler in the country.
As a trawler, the Castle-class is useless and obsolete as a warship. However, it can be used for practically any purpose imaginable, aside from combat. For example, fishing.