Sveti Georgi Rotunda

Regarded as the oldest preserved building in Sofia, the Sveti Georgi Rotunda, in the courtyard between the Sheraton Hotel and the Presidency, dates from the 4th century AD. This circular Roman structure, also known as the Church of St George, was largely rebuilt in the 6th century after being knocked about by invading Huns and in the 16th century was converted into a mosque. It was badly damaged by bombing during WWII and only fully opened to visitors again in 1998 after much restoration.

The murals inside were painted on three layers between the 10th and 14th centuries. Inside the entrance there's a small explanation in English about the church. You're also allowed to wander around the unlabelled Roman ruins behind the church.