This enormous 5th-century structure honours Thessaloniki’s patron saint. A Roman soldier, Dimitrios was killed around AD 306 at this former Roman bath site by order of Emperor Galerius, infamous persecutor of Christians. The martyrdom site is now an underground crypt. Dimitrios’ relics occupy a silver reliquary inside. The Ottomans made Agios Dimitrios a mosque, and plastered over frescoes that were again revealed after the 1913 Greek reconquest. While the city’s devastating fire of 1917 was very damaging, five 8th-century mosaics survive, spanning the altar.