Rome may have lots of crowds, but only three essential sights are difficult to see without advance reservations: the Colosseum, the Vatican Museums, and the Borghese Gallery. And each of these lets on-the-ball visitors make reservations in advance online. A 10-minute walk from the tourist-infested Colosseum is a free-standing ruin of nearly equal vastness — the impressive Baths of Caracalla. It’s unique in that for 1,500 years it has stood as a ruin, with nothing built around or on top of it. Today, with a good imagination, you can picture Rome at its zenith. Join tour guide Francesca Caruso and me for a stroll through the baths, 1,800 years ago.