On 23rd March 1943, Union Castle’s Windsor Castle was attacked and sunk some 110 nautical miles north-west of Algiers, while in Convoy KMF-11. This was a fast convoy of nine merchant vessels and 10 escorts, heading from the Clyde to Algiers. She was hit by an aerial torpedo around 2.30am, but did not sink until some 13 hours later. This gave the escorting vessels ample time to rescue the 2,699 troops and 289 crew aboard: only one crewman died in  the attack. Subsequent descriptions stated that the troops lined up on deck almost as if on parade, and quietly waited for orders to enter their respective rafts or transfer to the escorting destroyers. There was no panic or confusion at all; the troops were subsequently taken to Algiers and landed, albeit without any of their kit or equipment.

Originally ordered from Harland & Wolff, in Belfast, along with her sister, Arundel Castle, the order for Windsor Castle was transferred to John Brown’s yard on the Clyde, because of wartime shortages of materials. She was the last of the four-funnelled liners to be built. Launched on 9th March 1921, she was handed over in April 1922. In 1937 both Arundel Castle and Windsor Castle were rebuilt by Harland & Wolff: the forepart was lengthened and modernised, new turbines were installed and two fatter raked funnels replaced the four thin stovepipes. Both were popular ships on the run to South Africa.

In 1939 both Windsor Castle and  Arundel Castle were requisitioned for use as troop transports. They were painted in a simple plain grey camouflage scheme. After several successful convoys, including one in which a bomb hit her but failed to explode, her luck had run out.