Fort William Rufus was an artificial island made of concrete, situated west over Keehi Lagoon toward the entrance of Pearl Harbor, Sandwich Islands. Its fire-power consisted of two twelve-inch guns and numerous three-inch anti-aircraft guns. It was constructed by the British in order to protect the Harbor from enemy attack. Because of its concrete casing, it was known universally by its nickname, the Concrete Battleship.
In 1914, the US Navy invaded the islands. Oahu and all of its forts fell in fairly short order, leaving only the garrison at Fort William Rufus. So long as the fort was there, the U.S. Navy could not use Pearl Harbor. U.S. Admiral George Dewey offered full military honors for surrender, but the garrison refused, forcing the US to take the fort.
Fort William Rufus' weakness was that while it was invulnerable to direct assault, it had no anti-personnel defenses. After taking out one of the three-inch anti-aircraft guns on the fort's stern, a combined US Marine, sailor and combat engineer force successfully approached it, and poured a volatile mixture that was two parts heavy diesel oil, and one part gasoline, down the ventilation shafts. After attaching explosives to the shafts, they withdrew and blew them up, destroying the fort in a titanic blast.
William Rufus was an 11th-century king of England. He was one of England's most ineffective and unloved kings, making him an unlikely namesake for a proud fortress. The name seems to be Harry Turtledove's inside joke on OTL Oahu's Fort Shafter, which is named after 19th-century U.S. Army General William Rufus Shafter.