Serdica Amphitheater is a large amphitheater in the center of Sofia that once served the ancient Roman settlement of Serdica. It was probably built in the 3rd and 4th centuries sometime during the reign of Emperor Diocletian, and was completed during the reign of Emperor Constantine the Great. It represents the largest amphitheater in the entire Eastern Roman Empire and is certainly the largest in what is now Bulgaria; It is located outside the ancient city walls, and much of it is still preserved, and was used in regular battles between gladiators and wild beasts.