the start for Byzantium is as follows.
-ally serbia/wallchalia
-dow albania (dont call allies in)
-send your fleet to blockade the port, once the warscore ticks to 25% sue for peace and vassalise them.
-this will put you in a war with Ottomans, instantly peace them and give up albania and athens as vassals.
-you now have a 5 year truce, suck up to the big boys (austria,Hungary,poland - i didnt worry about lithuania, 9 times out of 10 they end up in a PU with poland so as long as poland joins lith will too)
- the war with the ottomans will have give you CBs against serbia/wall because they would not have joined you. destroy them.
- try and flip some of the islands. Tribenzoid is a good target for conquest too as you have a core
- the biggest threat to a Byzantine game is the second inevitable war with the ottomans. its not about winning is about survival. keep you army in the balkans so they dont get slaughtered in Constatinople.
-wait for your allies to come help you and just try and score enough wins and kill enough of their doomstacks to get a white peace. if you survive this you are pretty much home and hosed. after the second war they generally start shit fights with the mamalukes and you can hit them when they are down.
-go way over your naval force limit because you can keep them penned in asia, after the second war, and have no opposition in europe. it doesnt cost much for naval forces i think I am currenlty about 20 ships over mine and it costs me an extra 1 gold a month.
- dont push things too far, if you get into a war and it starts to turn nasty get out with what you can. the ottomans are a beast and things can go badly fast.
-if you can break their alliance with crimea it helps.
- tribenzoid + giving up serbia/wall/bosnia as vassals can be a good way to get peace out of the OE for the second war, also you have a core on tribenzoid so you can take it back later. you almost cannot win this war its more about not losing as i said above.
-you can still break the AI a bit with the bosborus crossing. if you move your fleet into the straights every time they try and cross they will stop crossing and start walking around thru crimea. you then run your fleet back to port before it gets destroyed by the stronger OE fleet. the AI often gets half way to around georgia then turns to come back across the straight. rinse and repeat.
in my current campaign they are a spent force owning only the coast of asia minor and their capital in europe. I finally havea decent forcelimit and ive begun expanding into italy whilst at peace with the ottomans.