After the conquest of the Southern Black Sea Coast by the Roman commander Lucius in 72 BC, the Emperor Vespasian founded east of the Thracian settlement of Develt or Debelt the colony of Deultum. Before Deultum was founded, there was an ancient Thracian settlement on its site, before the beginning of the new era. In the 70s of the 1st century, under the rule of the Roman Emperor Titus Flavius Vespasian, veterans of the VIII Augustus Legion settled there. Deultum was the only colony of free Roman citizens on the territory of today's Bulgarian lands. The town was built according to the Hippodamus system, which means a rectangular street network oriented along the cardinal directions, with a very good water supply and sewerage. There was also a harbour next to the ancient city, which was located on today's Lake Mandra.
The history of Deultum continued its ascending development and over the next three centuries the town became one of the richest in the surrounding area. In the 2nd century AD, during the reign of Emperor Marcus Aurelius, 12-metre high walls were built around Deultum. The town had a huge square, a local senate, Roman baths visited by women before midday and by men after midday. The population was extremely wealthy. Each citizen was given 50 iugera of land, equivalent to about 12 500 decares. Their lands stretched between the Balkan Mountains and the Bosna Ridge, the northernmost ridge of the Strandzha Mountains. In fact, these were the domains of the colony itself. It was directly subordinate to the Roman Emperor, who often came to these lands. The strategic location of Deultum, its impressive town planning and its noble inhabitants earned it the nickname Little Rome in Thrace.
In 812, Khan Krum captured Develt (Deultum) and expelled its population north of the Danube. This way the town became entirely Bulgarian. Historians believe that this area is strongly connected with the Conversion in Christianity of Bulgaria in 864. According to some hypotheses, King Boris himself was baptized in Deultum. Although this is only conjecture, the fact is that it was through the former Roman colony that the conversion delegation passed on its way to Pliska. The reason? It’s on the shortest route between Pliska and Constantinople.