The oldest building in Sofia, dating back to the 4th century AD. This small but remarkably preserved Roman rotunda was originally a bathhouse, then converted to a church. Inside, you'll find three layers of medieval frescoes, the oldest dating to the 10th century. Surrounded by the remains of Roman Serdica, it sits in the courtyard of the Presidency.
Don't miss the changing of the guard at the Presidency building next door. The frescoes inside are stunning - look up!