This is a must-visit in Sofia. It's the oldest church in Sofia, having been built in the 4th century, and it is exceptionally well preserved for such an old building. It can be a little difficult to find (it's inside the presidential complex behind a security gate), but if you take the free Sofia walking tour they'll take you right by it. Outside the church are some more ruins from the ancient Roman city of Serdica. It's fairly small, and I would try to read up on the history and frescoes before entering as it will make the experience more worthwhile.
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