The oldest building in Sofia — a small red-brick rotunda from the fourth century, hidden in a courtyard behind the Presidential Palace and the Sheraton hotel. It was built as a Roman bath, became a church under Constantine's successors, then a mosque under the Ottomans, and is now Orthodox again. Working services still happen here.
Inside, the dome holds three layers of medieval frescoes, the earliest from the tenth century. Around the rotunda are Roman ruins from ancient Serdica: an Imperial palace, hypocaust heating systems, and stretches of marble pavement. Access is free; the surrounding archaeological excavations are open without a ticket.