Basil II Bulgaroctonus

The Eastern Empire in its early years would have been extremely difficult to unify. Not only are there diverse cultures and peoples, the geography ensures that the Byzantines would have had to keep a standing navy in prime shape at all times in order to link Africa with Greece, which it obviously failed to do. If Islam had not risen, I supposed the Byzantines would have had to continue their power struggle with some other Middle-Eastern empire (which would certainly have arisen in the place of Persia), or vie with Western Europe. Either way, the empire being located at the borderline of Europe/Asia would not have lasted much longer than it did. My personal feeling is that Islam kept Western/Eastern Europe from going at each other's throats immediately by presenting itself as a unified danger. The religious differences between Orthodoxy and Catholicism would have easily been enough to start war had not the early Arabic states kept them in an uneasy alliance.

This "What if" is a long shot. War never turns out the way it's "supposed" to. We'll never know if Western Europe would have unified and faced the common threat, or if they would have fallen piece by piece. If the Ottomans wanted to attack Rome, they would have had to face the Genoan and Venitian navies. At the Battle of Otranto they were clearly defeated. They were also repulsed at the Siege of Vienna. Does that mean they wouln't have been able to take Rome if they had persisted? Not necessarily, but the fact that they chose to abandon the Western European campaign says something of their ability to strike deep into the heart of the West.

It is highly unlikely that any resistance was given at all. Constantinople at the time was heavily undermanned, and as the capital is the more important of the two I believe any remaining troops (if there were any) would have been recalled to the capital. I usually trust Wikipedia, but if there was a defense it was probably only a gesture. I've never heard of a siege in this town, but I may be wrong.