With a history spanning more than two millennia, Thessaloniki reigned alongside Constantinople as a capital of the Byzantine Empire. So numerous are the outstanding Paleochristian and Byzantine monuments that as a group they have been designated a UNESCO World Heritage Site. Don’t miss the ornate church and sacred crypt of St. Demetrius, the city’s martyred patron saint, or the palace complex of Galerius and the mosaics in the Rotunda. The Archaeological Museum holds even more ancient treasures.