Basilica di Santa Prassede in Rome

You're standing on Via di Santa Prassede, a narrow little street just a stone's throw from the huge Santa Maria Maggiore. Look at that modest side door. You could easily walk right past it, but behind it lies one of Rome's great treasure chests. You can't see the real façade from here. It sits behind a square forecourt, with a Romanesque gateway on Via di San Martino ai Monti, and that gate is usually shut. So everyone slips in from the side. Glance up, too: the bell tower rises above the left transept and dates from the late thirteenth century.