Looks like you failed the early diplomacy game. Hungary is usually a very good ally, the Kievan Rus aren't as good but work in a pinch.
Now then, you are the Byzzies. Your game is easy mode just from that. For future stuff, ally or no ally, you want a sizeable garrison on your every border. (If the zones boundry touches another zone that is not yours, put 4-5 units minimal. I use peasants for this almost exclusively. I love peasants.)
Other than your boned position, the only thing I would have done differently is ignore Athens. Go straight for the Sicilians in Corinth and kick their shins. Corinth is one of the better castles at the start of the game. You want it. It'll fuel your offensives for just about everything just out of sheer awesomeness. You can just ignore Athens, with all those rebels (and the rebel army north of it) it won't fall any time soon.
Seriously, take Corinth and fortify yourself in Europe first, get peace deals and the like. After that you finish that, murder the ottomans and peace out with the Crusaders. Don't go further to the East because the mongols are mean, let them get to you first. After the Mongols are finished, take over all the middle east. By then, Europe will probably start getting better troops but you will have so much money and production...
Edit: On BGR, I wanted to try it. I just don't understand half of the things it adds despite reading the manual. The supply and victory systems? Gotcha. The war council, and zeal, and who can recruit and who cant, etc. Nope.
