Serdica Amphitheatre was part of the ancient Roman city of Ulpia Serdica, now Sofia, the capital of Bulgaria. Discovered in 2004 and the subject of excavations in 2005 and 2006, the ruins of the amphitheatre lie on two adjacent sites in the centre of modern Sofia. The amphitheatre was built in the 3rd–4th century AD on top of a 2nd–3rd century theatre, which had been ravaged by the Goths. However, the amphitheatre remained in use for less than a century and was abandoned by the 5th century.
The Amphitheatre of Serdica was among the biggest in the eastern part of the Roman Empire and the largest in what is today Bulgaria. It lay outside the city walls of Serdica and hosted fights between gladiators and wild beasts, which were advertised at the entrance of the city.
References:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.