The siege of 626 failed because the Avars did not have the patience or technology to conquer the city. Though the Persians were experts in siege warfare, the walls of Constantinople proved to defend easily against the siege towers and engines, amongst the reasons being that the former could not move their siege equipment to the European side of the Bosphorus (which was heavily guarded), where their Avar and Slavic allies were initially stationed. Furthermore, the Persians and Slavs did not have a strong enough navy to ignore the sea walls and establish a channel of communication.