Shahrbaraz, along with a smaller army, later managed to slip through Heraclius' flanks and bee-lined for Chalcedon, the Sasanian base across the Bosphorus from Constantinople. Khosrow II coordinated with the Khagan of the Avars so as to launch a coordinated attack on Constantinople from both European and Asiatic sides. The army of Shahrbaraz stationed themselves at Chalcedon, while the Avars placed themselves on the European side of Constantinople and destroyed the Aqueduct of Valens. Because of the Byzantine navy's control of the Bosphorus strait, however, the Sasanians could not send troops to the European side to aid their ally. This reduced the effectiveness of the siege, because the Sasanians were experts in siege warfare. Furthermore, the Sasanians and Avars had difficulties communicating across the guarded Bosphorus—though undoubtedly, there was some communication between the two forces.