Along with the Colosseum, the Pantheon is one of Rome's iconic sights. A striking 2000-year-old temple (now a church), it is the city's best-preserved ancient monument and one of the most influential buildings in the Western world. The greying, pock-marked exterior might look its age, but inside it's a different story and it's an exhilarating experience to pass through its towering bronze doors and have your vision directed upwards to the world's largest unreinforced concrete dome.

Its current form dates from around AD 120, when Emperor Hadrian built over Marcus Agrippa's original temple (27 BC) – you can still see Agrippa's name inscribed on the pediment. Hadrian's temple was dedicated to the classical gods – hence the name Pantheon, a derivation of the Greek words pan (all) and theos (god) – but in AD 608 it was consecrated as a Christian church. During the Renaissance it was much studied (Brunelleschi used it as inspiration for his Duomo in Florence) and became an important burial chamber. Today you'll find the tomb of Raphael, alongside those of kings Vittorio Emanuele II and Umberto I.

However, the real fascination of the Pantheon lies in its massive dimensions and extraordinary dome. Considered the Romans' most important architectural achievement, it was the largest dome in the world until the 15th century and is still the largest unreinforced concrete dome ever built. Its harmonious appearance is due to a precisely calibrated symmetry – its diameter is exactly equal to the Pantheon's interior height of 43.3m. Light enters through the oculus, an 8.7m opening in the dome that also served as a symbolic connection between the temple and the gods. Rainwater enters but drains away through 22 almost-invisible holes in the sloping marble floor.

Somewhat the worse for wear, the exterior is still imposing, with 16 Corinthian columns (each a single block of stone) supporting a triangular pediment. Rivets and holes in the brickwork indicate where the original marble-veneer panels were removed.

Thanks to its consecration as a church in the 7th century, the building was spared the Christian neglect that left other structures to crumble, although it wasn't entirely safe from plundering hands. The gilded-bronze roof tiles were removed and Bernini used bronze from the portico for the baldachin at St Peter's Basilica. Thankfully, the original Roman bronze doors remain.