In 1882 the government of the then Ottoman province of Eastern Rumelia ordered a complete chemical analysis of the thermal mineral waters at Hisarya, a small town at the southern foot of the Sredna Gora mountain. The town boasts 22 hot mineral springs, which at the time were enjoyed largely by the locals. 

The analysis produced astonishing results: The hot mineral water at Hisarya was found great for the treatment of a range of kidney, bone, skin and eye diseases. Hisarya soon became a hotspot of the growing Bulgarian spa and water-treatment tourism, a status it has retained to this day. 

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Yet this was not the first time when people recognised the healing power of Hisarya's mineral springs and put it to use. In the 1st Millennium BC the ancient Thracians who inhabited the region were also aware of the water's curing capacity. When the Romans – unabashed aficionados of spa pleasures – took over the region in AD 44-46, it was only a matter of time for a settlement to appear by the springs and to begin attracting people eager to have a dip in them. 

A settlement was born and it grew. By 293 it was big and important enough for Emperor Diocletian, who himself tried and enjoyed the local springs, to promote it to the rank of a city and to give it his own name, Diocletianopolis, the City of Diocletian. The name Hisarya appeared later, in Ottoman times, a derivation from the Turkish word hisar, or citadel, which reflects the mighty ruins of the ancient fortifications. 

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Diocletianopolis was a typical late Roman city. Situated in the plain and with a quasi-rectangular plan, it was protected from the intensified invasions of the Barbarians with a 44-tower strong wall of bricks and stones – the wall was 2,327 m long and 11 m high, and had massive gates rising up to 13 m. Inside, a net of parallel streets ran along mansions and smaller homes. When Christianity became the official religion in the empire, a number of basilicas were built, some over the ruins of earlier buildings, including military barracks. Archaeologists have identified at least ten basilicas in Diocletianopolis, one of which is an architectural curiosity. It had two naves, a structure outside the contemporary one, which prescribed an odd number, usually one or three. Entertainment for the citizens was provided at the local amphitheatre, but given that it was operational during the Christian era, the shows most probably did not involve the spilling of blood. 

Understandably, the public baths of Diocletianopolis were bigger and richer than was habitual for a city of this size. They spread over 2,000 sq m, and were heated with the 51-degree-hot waters of the Toplitsa spring. Inside, the baths were decorated with marble, and apart from the rooms with different temperatures typical for Roman public baths, there were also two 15 m by 5 m pools as well as special rooms for massages and water treatment. Close by was a nympheum, a sanctuary to the deities of the springs. 

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Diocletianopolis survived the Hun attacks in the mid-5th Century, and although sections of its fortifications were initially destroyed, they were later rebuilt. Life in the city ceased at the turn of the 7th Century, killed or driven off by the continuous attacks of Avars and Slavs. Later the city revived as an early mediaeval settlement, and people have been living in it ever since. Unlike many ancient cities which was lost under the impact of uninterrupted inhabitation across the ages, a lot of Hisarya's Roman past has survived. In the heart of today's town, among the early 20th-Century villas, Communist-era sanatoriums and modern spa hotels, a number of Roman ruins lurk. 

The fortification walls of Diocletianopolis are the city's defining feature, and are one of the best-preserved late Antiquity fortresses in Bulgaria. The most imposing of the gates is the eastern one, still called Kamilite, or The Camels, because of its massive hunch. It appears on almost all advertising materials for the city and also on the bottles of the local mineral water, which you can buy all over Bulgaria.

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The core of the ancient city is concentrated and exhibited in situ at the so-called Momina Salza Park. There, among greenery and mid-20th Century gazebos and sculptures, one can walk along the finely preserved ruins of the ancient baths and shops, a residential building and the amphitheatre.

The necropolis of Diocletianopolis was outside its walls, and featured five late Antiquity tombs. The most interesting of them, and the only one open for visits, is at the Slaveev Dol Park, about 300 m southeast from the fortress wall. It was built in the latter half of the 4th Century, and has a corridor, a staircase and a chamber. Few traces of murals are preserved on the walls, but in the burial chamber a beautiful ornamental mosaic has remained intact from the days when Hisarya was called Diocletianopolis and Romans enjoyed its mineral springs. 

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