Pantheon

Two thousand years old, free to enter, and still the largest unreinforced concrete dome on the planet. The Pantheon is the building that makes engineers and architects stop talking mid-sentence. Walk through the massive bronze doors - originals, by the way - and look up at the coffered dome with its oculus, a 9-metre hole in the ceiling that is the only source of light. When it rains, the water falls straight through and drains through nearly invisible holes in the slightly convex floor. That's Roman engineering from 125 AD, and nobody has improved on it since.

The interior is one enormous room, perfectly proportioned: the height to the oculus equals the diameter of the dome (43.3 metres). This was deliberate. The Romans designed it so the space would feel both vast and harmonious, and it works. The light beam that enters through the oculus moves across the interior like a slow spotlight - at noon it's at its most dramatic, hitting the floor in a near-perfect circle. Morning visits between 8:30-9:30 AM catch the beam sweeping across the coffered ceiling, which is arguably even more beautiful.

Entry is free but you need a timed reservation since 2023. Book on the official site a few days ahead - it costs nothing, takes 30 seconds, and saves you from the walk-up queue that can reach 45 minutes. The reservation system was controversial but it's actually improved the experience because they now limit the number of people inside at any time.

Raphael is buried here, in the third chapel on the left - most people walk right past it. The building started as a temple to all the gods (pan = all, theon = gods), was converted to a church in 609 AD, and has been in continuous use ever since. The piazza outside has the usual overpriced tourist cafes - walk two minutes south to Sant'Eustachio Il Caffe for what many Romans consider the best coffee in the city (€1.10 standing at the bar).