Sant'Agnese fuori le Mura & Mausoleo di Santa Costanza

One of Rome's most important and least-visited early Christian complexes — a 7th-century basilica, a 4th-century mausoleum, and catacombs, all grouped on Via Nomentana outside the Aurelian Walls. The Basilica of Sant'Agnese contains one of the finest Byzantine gold-ground mosaics in Rome: Saint Agnes flanked by two popes, dating to around 630 AD, blazing with gold above the altar. More remarkable is the adjacent Mausoleo di Santa Costanza — a circular 4th-century mausoleum built for Constantine's daughter Constantina, its low domed ambulatory covered in the oldest surviving Christian mosaics anywhere in the world (c. 340 AD): vine-scroll, harvest, and geometric patterns of extraordinary delicacy. Bring a €0.50 coin to illuminate the mosaics. Consistently cited by Lonely Planet, Rick Steves forums, and Rome Actually as one of the city's greatest hidden treasures.