The Pantheon, which was a temple built for all of the gods, was the first large dome.
Concrete allowed the Romans to construct large-scale buildings. Many of the famous Roman monuments still stand today. Because the empire covered such a wide territory, their building skills covered a large area too and were very big. (Hence, the Romans are considered to be the "great builders of the world").
Baths were vast enclosed structures that contained libraries, lecture rooms, gymnasiums, shops, restaurants, and pleasant walkways. They had rooms with progressively cooler water: a Calidarium, a Tepidarium, and a Frigidarium. The largest Bath in Rome was built by Caracalla; the vaulted ceilings were up to 140 feet high.