Sure, we saw some aquaducts (also an impressive engineering feat), plus the entire city of Pompei with what was left of it’s streets, shops, homes and brothels. But in terms of scale and technological achievement, the Colosseum is really in a class by itself.
And in any Renaissance city, the local duomo (cathedral ) is likely to be one of, if not the largest structure by several orders of magnitude.
I think pretty much everyone from all over the world likes to see the Colosseum when they go to Rome.