Mausoleo di Costanza in Rome

You're standing at Santa Costanza, just off the Via Nomentana, a short way outside the old walls of Rome. Start with the shape. It's round, made of brick, with a tall drum topped by a shallow dome. It doesn't look like an ordinary church, and that's because it wasn't built as one. This was a mausoleum, put up in the fourth century for Constantina, a daughter of the emperor Constantine the Great. It's one of the best preserved buildings from Late Antiquity anywhere in Rome.