The Colosseum

Yes. Entry is by timed slot through the official system and the slots sell out, especially in spring and summer and especially for the first entries of the day. Turning up without one means either a long queue for whatever is left or no entry at all. Book as early as you reasonably can, and note that the arena floor and the underground hypogeum are separate limited add-ons that go before general admission does — if you want either, choose it at the point of booking rather than at the gate.

The standard combined ticket covers all three, and they sit next to each other, so it makes sense to plan them together. What varies is the validity window and how many entries you get, and that has changed more than once, so check the terms when you book rather than relying on what was true on a previous trip. Practically, the Colosseum takes two to three hours and the Forum and Palatine together take at least as long again, so attempting all three in one summer afternoon is more than most people enjoy.

If you are interested in how the building actually worked, yes — more than the arena floor is. The hypogeum is the two-level warren of tunnels, cages and lift shafts beneath what was once a wooden floor covered in sand, added under Domitian, and it is where the machinery of the spectacle sat. Seeing it turns the Colosseum from a large ruin into an understandable piece of engineering. It is a restricted-access add-on with limited places, so it has to be booked at the outset.

Two to three hours for the Colosseum itself, including security screening before the ticket check, time on the tiers and a walk round the outside. Add half an hour if you have the arena floor, and about an hour for the underground. The Forum and Palatine on the same ticket need two to three hours more, so a thorough visit to all three is close to a full day and is better split across two if your ticket allows it.

Two reasons, and neither is neglect in the way people assume. Earthquakes, particularly a severe one in the fourteenth century, brought down much of the southern outer wall. Then for centuries the fallen stone — and a good deal of stone that had not fallen — was carted off and reused in other Roman buildings, along with the metal clamps that held the blocks together, which is why the surviving stonework is pocked with holes. The result is accidental but useful: the building is now half a cross-section, and you can read its structure from the ground up in a way you never could have when it was whole.

The first entry slot of the day, particularly between June and September. The site is almost entirely unshaded, the stone holds heat, and by midday in a Roman summer it is genuinely unpleasant as well as crowded. Late afternoon is the next best option outside high summer and gives warmer light on the travertine. In winter the crowds thin considerably and the low sun through the arches is the best light the building gets all year, which is a reasonable trade for the cold.