Philippolis Fortress, Ancient fortress in Plovdiv, Bulgaria.

Philippolis Fortress is an ancient fortification spread across multiple hills in Plovdiv, featuring strategic towers and thick defensive walls. The ruins form a ring around the city's historical center and remain accessible in many sections.

The walls date from the 4th century BCE to the 7th century CE, with five building phases identified. In 172 CE, under Marcus Aurelius, an inscription found near the East Gate records new walls enclosing about 78 hectares. After Gothic invasions the system was rebuilt, and Justinian I refortified only the Three Hills, where the best-preserved curtain and round corner tower stand near the church of Sts. Constantine and Helena.

The fortress displays construction styles left by Macedonians, Romans, Byzantines, and Ottomans as you walk along its walls. Each period used different building techniques and stonework that remain visible in the structure today.

The best-preserved stretch of the Justinianic wall, including a high curtain section and a round corner tower, stands next to the Church of Saints Constantine and Helena. Other fragments can be seen on the north and northwest slopes of Nebet Tepe and beneath Dzhumaya Square beside the Ancient Stadium.

The best-preserved stretch of Justinian I's sixth-century wall stands near the church of Saints Constantine and Helena, where a high section of curtain wall and a round corner tower survive; the other towers were triangular, and a second outer wall, a proteichisma, ran in front.