The Painted Hall, Greenwich

They call this Britain's Sistine Chapel, and for once the comparison isn't hyperbole. You're about to walk into three thousand seven hundred square metres of Baroque painting — ceiling, walls, and everything in between — covering every surface of a single enormous room. Two hundred figures. Kings, queens, mythological creatures, celestial beings, naval heroes. It is staggering.

The artist was James Thornhill, and it took him nineteen years. He started in seventeen oh-seven and finished in seventeen twenty-six. The pay structure was beautifully absurd: one pound per square yard for the walls, three pounds per square yard for the ceilings, because painting overhead is harder. The total bill came to roughly what would be one point four million pounds today. Thornhill was knighted for his efforts, making him the first native-born English painter to receive that honour.

But here's the punchline. The Painted Hall was designed as a dining room for retired sailors at the Royal Hospital for Seamen. Injured veterans of the Royal Navy were supposed to eat their meals here, surrounded by all this grandeur. Except the authorities took one look at the finished room and decided it was far too beautiful for retired sailors to eat in. So the men it was built for were redirected to a plainer room, and the Painted Hall became a tourist attraction. A dining hall too grand for diners.

In eighteen oh-six, Lord Nelson's body lay in state here for three days before his funeral at St Paul's. Tens of thousands filed past to pay their respects. The coffin was placed beneath the painted ceiling that the common sailors were never allowed to eat under.

The building itself was designed by Christopher Wren and Nicholas Hawksmoor. Because in Greenwich, even the rooms nobody's allowed to use are masterpieces.