Colosseum

Fifty thousand spectators, four storeys of arches, and an engineering system so advanced they could flood the arena floor for mock naval battles. The Colosseum doesn't need a sales pitch - but standing inside it, on the edge of what was once the arena floor, looking down into the exposed tunnels where gladiators and wild animals waited to be lifted into the fight, is a different experience from any photo. The scale is the thing. Your brain can't quite reconcile that this was built in 80 AD.

The €18 combined ticket covers the Colosseum, Roman Forum, and Palatine Hill for two consecutive days - which is the smart way to do it. Use day one for the Colosseum, day two for the Forum and Palatine. Book online at least a week ahead; walk-up tickets exist but the queue can stretch to two hours in peak season, and the guys in gladiator costumes outside charging €10 for a photo are just the start of the annoyances if you arrive unprepared.

The €24 underground tour adds the arena floor and the hypogeum - the network of tunnels, lifts, and trap doors beneath the surface. If you can get a slot (they sell out fast), it's worth every cent. Walking through the corridors where animals were kept before being hoisted into the arena on mechanical elevators is genuinely chilling. The standard ticket gets you the first two levels; the third level opened a few years back and has the best panoramic views of the interior, plus you can see the Forum from above.

Timing is everything. Gates open at 9 AM and by 10:30 the tour groups arrive in waves. Enter from the Via dei Fori Imperiali side entrance (shorter line than the main Piazza del Colosseo entrance). Go straight to the third level for photos while the light is good, then work your way down. Late afternoon after 3 PM is the other sweet spot - the crowds thin and the golden hour light through the arches makes the travertine glow. Bring water. There's almost no shade inside.