 On 29 June 626, the Avars and Persians began a coordinated assault upon the walls.The Byzantine defenders had 12,000 well-trained cavalry troops, who were likely dismounted, facing roughly 80,000 Avars and Sclaveni (Slavs whose land was controlled by the Avars).Because the Persian base in Chalcedon had been established for many years, it was not immediately obvious that a siege would take place.