As we all know the second siege of Constantinople in 98 AH ((717 AD)) ended with a narrow but decisive Umayyad victory, this marked the end of the Roman Empire as its last remnants were seized one by one by either the Muslims, the Lombards or the Bulgars, even through some important stronghold resisted for several years.
The fall of the city is also considered the official end of Ancient History by most historians.
But what if Constantinople didn't fell? The city was heavily fortified and many say that the Arabs won mainly due to sheer luck.
What could have happened to the Roman Empire and to Europe? And what about Islam and the Caliphate?