Designed in 1804, the Grand Shaft, at the Western Heights of Dover, is a triple helix, the only such staircase in Britain. As with Vatican City’s Bramante Staircase, this design accommodates a large number of passengers while minimizing interference among them — using it, a large number of troops might quickly descend the 140 feet from the heights to the town below, while others might even ascend at the same time, using a different spiral.

In 1812 a Mr. Leith of Walmer rode his horse up the stair for a bet, and local legend has it that during Victorian times the separate spirals were assigned to “officers and their ladies,” “sergeants and their wives,” and “soldiers and their women.”

(Thanks, Dave.)