Byzantine emperors desired stability and security in the peripheral regions of the empire so as to continue controlling and exploiting the productive lands which provisioned the principal cities, most importantly Constantinople; these also yielded tax revenues to support the apparatus of government. In the Balkans the vital regions were the rich lands of Thrace and the hinterland of Constantinople in the east, and Thessaly and the lands around Thessaloniki in the west. Security required direct supervision of major communication routes, by land and water, and of strategic cities across the peninsula, but only a stabilising influence in the mountainous interior, the north-eastern plains and the north-western littoral. Control of the Black Sea ports between Constantinople and the lower Danube, notably Anchialos, Mesembria and Varna, was considered essential, as was command of the major mountain passes through the Haemus mountains. Minor paths remained in the hands of locals, largely Vlachs, whose allegiance was assiduously cultivated.