Cross posted from Monstrous Beauty.
The Basilica of Santa Sabina is one of the oldest surviving early Christian basilicas. A few other basilicas are older (St Paul's Outside the Walls, St. Peter's, The Lateran) but they have all been substantially rebuilt or modified. Only St. Sabina's exterior remains close to the appearance of an early basilica.
Santa Sabina was built about a century after Constantine legalized Christianity. Although, it was not one of the original churches built under Constantine's patronage, but was built in the same style. The exterior is rather severe, without any external decoration. The clerestory windows above the aisles are filled with selenite rather than glass and allow large amounts of light into the interior. The interior was originally decorated with mosaics, but those are now lost. The original 5th century wooden door, carved with scenes from the Bible still exists.
Santa Sabina now serves as the mother church for the Dominican Order.
Image from Wikipedia.