She was laid down on the River Tees in the last year of the First World War, one of the standard cargo steamers the Shipping Controller ordered in bulk to replace the merchant tonnage that German U-boats had sent to the bottom. The Controller named them all with the prefix War; sources disagree as to whether this one was launched as War Acacia or War Ocean. Either way, the armistice came before she sailed for the government, and she was sold to the Westcott & Laurance Line - a Liverpool subsidiary of the giant Ellerman Lines - who renamed her Castilian. She was the second of three Ellerman ships to bear that name; the first, the former Umbilo of Bullard, King & Company, had been torpedoed and sunk by U-46 northwest of Ireland in 1917. The third, built in 1955, eventually ended her days as the Maldive Freedom. The middle Castilian served twenty-four years between Britain, Gibraltar, the Mediterranean, and the North Atlantic convoys.

Through 1940, 1941, and 1942, Castilian crossed and recrossed the Atlantic - westbound in OB and ON convoys, eastbound in HX and SC convoys with cargoes of Canadian and American war material. By the start of 1943, with the U-boat threat in the Western Approaches at its peak, the planners decided that some smaller cargoes were quicker dispatched alone. On 11 February 1943 Castilian left Liverpool unescorted, bound for Gibraltar with what her manifest described as munitions: shells, fuses, propellant charges. She rounded the northwest tip of Anglesey in the dark of the following night. The exact navigational error is no longer recoverable, but the result is on every chart: she ran onto the rocks somewhere off the Skerries reef, took on water faster than her pumps could clear, and sank by the head. Reports of the crew evacuation are brief and incomplete. There were no significant casualties; the ammunition in the holds did not detonate.

For decades after the war the wreck was known only to local fishermen and a handful of sport divers, who reported intact crates of small-arms ammunition lying on the seabed inside the broken hull. In 1987 the Royal Navy sent a clearance diving team to Fydlyn Bay on the Anglesey mainland, where unexploded ordnance had been washing ashore for years; the divers spent several months recovering rounds believed to have drifted in from the Castilian. The wreck itself was then formally designated under Section 2 of the Protection of Wrecks Act 1973, the section dealing not with historic vessels but with dangerous ones. The 500-metre exclusion zone has been in place ever since. The Castilian shares the dubious company of just a few other British shipwrecks - including the SS Richard Montgomery in the Thames Estuary, sunk in 1944 with 1,400 tonnes of TNT still aboard - that the law refuses to let anyone touch, for the simple reason that touching them might be the last thing anyone ever does.