The largest Roman amphitheatre ever built, the Colosseum is an extraordinary achievement of Roman engineering. It is also the only building in the world to feature what has been termed the ‘Composite’ order of column.
The dome of the Pantheon is the largest unreinforced concrete dome in the world, an extraordinary achievement for a building constructed in 26AD (or indeed at any point in history).
Santa Maria Sopra Minerva
Uniquely, this is the only Medieval Gothic church in Rome.