Under Omurtag (814 831, the southern Black Sea coast up to Debelt was recognized as Bulgarian. That happened in 815 with the signing of the 30-year peace treaty with Byzantium. The success of the Black Sea direction of Bulgarian policy was thus legalized.
After the stabilization of the seaside border up to Mount Strandzha, Kan Omurtag maintained the other important orientation of his foreign policy the orientation, started by Kan Krum, to the northwest towards the Slavs on the banks of Middle Danube, who were separated from Bulgaria by the Empire of the Franks. This is how the contemporaries perceived it (according to the so-called Fulden Chronicles — The Life of Charlemagne by Einhard):
In 827, “the Bulgarians sent troops in ships up the River Drava, plundered and burnt the land of the Slavs inhabiting Pannonia, drove away their kings and appointed Bulgarian governors”; in 829, “again the Bulgarians came in ships up the Drava, destroyed by fire some of our settlements near the river”.
The Bulgarian “admiralty” of the period is no doubt located in and identified with the powerful island fortress and port of modem Romanian Pakuiullui Soare Island that is about 20 kilometers to the east of the Bulgarian fortress and port of Drustar (present Silistra). To this very day one can see the deep traces in the stone blocks of the quays on the island, left by the ropes with which they moored the ships.
The building of this fortress harbor as an “admiralty” is due to its exceptional political and military strategic location on Lower Danube as an internal Bulgarian river. Drustar fortress harbor secured and protected the direct connections of the capital Pliska, and later of Preslav to the Bulgarian lands to the north of the Danube, which stretched to the Carpathians.
In addition, together with Drustar, this island fortress harbor controlled the trade along the Danube and advantageously blocked the invasions of enemy (mainly Byzantine) fleets trying to penetrate into the Danube from the Black Sea. That was done actively with ships and passively by means of long chains lowered in the water, which connected the two riverbanks.
Kaliakra was another important fortress harbor on the north Black Sea coast of Bulgaria during the 7th-8th-10th- 11th centuries. There the continuity between the Late Antiquity of Byzantium and the early medieval period of Bulgaria is documented excellently. This fortress harbor played an extremely important military strategic and overseas trade role for the Bulgarian state with its strong walls, first-rate harbor area and numerous all-purpose ships.