This charming round red brick church dates back to the 4th century, although it did not become a church till the 6th century. It is Sofia's oldest preserved building. Having undergone major restoration work, it was reopened to the public in 1998 for the first time in 70 years. Three layers of frescoes can still be seen, the oldest dating back to the 10th century illustrating the development of monumental painting through to the 14th century. Nowadays it often hosts small art exhibitions.
Services are held daily at 8.00; 17.00 and 21.00. The excellent website has a section called 'chants' where you can listen to recordings of Old Church Slavonic chants.