Trier's most famous landmark, this brooding 2nd-century Roman city gate – blackened by time, hence the name, which is Latin for ‘black gate’ – is a marvel of engineering, since it’s held together by nothing but gravity and iron clamps. In the 11th century, the structure was turned into a church to honour Simeon, a Greek hermit who spent six years walled up in its east tower. After his death in 1134, he was buried inside the gate and later became a saint.