Colosseum, Rome

The great amphitheatre of ancient Rome, opened in 80 AD, where up to 50,000 spectators watched gladiator fights and animal hunts. Even in ruins it's awe-inspiring. Tours can take you onto the arena floor and down into the underground passages where the animals and fighters waited.

Tickets are timed and sell out, so book on the official site well ahead; the standard ticket also covers the Roman Forum and Palatine Hill. Go early or late in the day. Beware of unofficial sellers outside.