In 46, Thrace was conquered by the Romans and transformed into a Roman province. Data from a Latin inscription originating from Hisarya testify that in 135-136, during the time of Emperor Hadrian, the area around Hisarya mineral springs was declared an imperial domain (Augustae) with its own borders.
This imperial domain had a special status and was directly subordinated to the emperor. As early as the middle of the second century, the construction of a large Roman bath with a nymphaeum (the sanctuary of the nymphs) began at the present-day “Momina salza” (“Maiden’s tear”) spring. The healing power of mineral springs has gained popularity throughout the ancient world. People from all over the vast Roman Empire came to be treated here.
In 293, Emperor Diocletian (284-305) declared his domain around the Hisarya mineral springs for a town called Diocletianopolis. The city was built according to all the rules of Roman urbanization with the typical for the Roman cities Hippodamian plan: wide, intersecting streets, water supply system for cold drinking water and for cooling the hot mineral water in the Roman baths.
In the 5th and 6th centuries, the Roman city of Diocletianopolis was in its prime. Then it grew beyond the walls of the fortress. The guidebook of the ancient author Hierocles from 528 mentions that during this period Diocletianopolis was the third largest city in the province of Thrace after the cities of Philippopolis (Plovdiv) and Beroe (Stara Zagora). In Diocletianopolis, some of the most prominent figures in the empire were healed. Solomon, vicar of the province of Thrace, who ruled during the reign of Emperor Mauritius (582-602), not only stayed here, but was also buried in one of the necropolises of the Roman city.
The archaeological excavations carried out at Diocletianopolis show that the town was surrounded by a strong fortress wall, which was built during the reign of the Roman emperor Diocletian (284-305). The wall is shaped like an irregular tetragon and surrounds an area of 30 hectares. The total length of the wall is 2327 m. In the 5th c. the defensive strength of the fortress from the north was reinforced by a second wall (proteichisma).
The city was entered through four main gates. The best preserved to this day is the western fortress gate. It consists of a vaulted passageway flanked by two rectangular towers. The passage was closed by a double door.
Of greatest architectural interest is the southern gate. This gate, like the western one, consists of a vaulted passage, but on the outside it is preceded by a larger arch, supported by two massive pillars. The south gate is also flanked by two rectangular towers. Recent studies at the southern fortification gate of Diocletianopolis have shown that at its top it did not end with an independent third tower (machicolation), but represented a continuous, two-level U-shaped battle platform, which was climbed through an internal five-shoulder staircase, built in the western frame of the gate. The broken brick arch preserved to this day on the ridge of the south gate is actually part of a decorative brick vault, in the middle of which there was an embrasure for direct firing at the enemy. The gate was closed with a double door, secured from the inside by a massive wooden beam.
Above the arch, the entire interior facade is decorated by a brick cornice that served as the base of a triangular pediment .
