The relations between the Bulgars and the local Slavs is a matter of debate depending on the interpretation of the Byzantine sources. Vasil Zlatarski asserts that they concluded a treaty but most historians agree that they were subjugated. The Bulgars were superior organisationally and militarily and came to dominate politically the new state but there was cooperation between them and the Slavs for the protection of the country. The Slavs were allowed to retain their chiefs, to abide to their customs and in return they were to pay tribute in kind and to provide foot soldiers for the army. The Seven Slavic tribes were relocated to the west to protect the frontier with the Avar Khaganate, while the Severi were resettled in the eastern Balkan Mountains to guard the passes to the Byzantine Empire. The number of Asparuh's Bulgars is difficult to estimate. Vasil Zlatarski and John Van Antwerp Fine Jr. suggest that they were not particularly numerous, numbering some 10,000, while Steven Runciman considers that the tribe must have been of considerable dimensions. The Bulgars settled mainly in the north-east, establishing the capital at Pliska, which was initially a colossal encampment of 23 km2 protected with earthen ramparts.