A large open-air archaeological complex right under Independence Square in the Largo, exposed during construction of the Serdika metro station. You walk through Roman streets, the foundations of the city gate, baths, and an early Christian basilica — all of it once buried under Communist-era pavement.
The site is accessible directly from the metro, free of charge, and surprisingly extensive: paved Roman cardo and decumanus, water channels, and the remains of merchants' houses. Information panels are in Bulgarian and English. The whole thing is open 24 hours.