Early -

Lots and lots of cheap troops while you build up economic infrastructure.

Fortify and defend Greece and Bulgaria as bulwarks against the west.
Turn the islands into ship factories and flood the eastern Med.
Do weapons and armour upgrades in Byzantium and Antioch for good troops.

Look east. Expand Eastward and take Trezibond at which point fortify and defend.
Look South. Take everything to egypt at which point fortify and defend.

Consolidate. Replace cheap troops with good, shunting the cheap ones to egypt to be used to push westward along North Africa.
Consolidate. Build Infrastructure in your now secure territory, including a good mix of function types depending on specialty or isolation.

Defend. Build up two quality armies of around 5,000 apiece in Trezibond and the territory to its south led by 5 star generals.
Defend: Build up two quality armies of around 2,000 apiece in Greece and Bulgaria led by 5 star generals.

Repulse the Mongols when they arrive, as many times as necessary, and then defeat them in their own territory (usually the province north of Trezibond).

The year is 1234 and you have smashed the Mongols, you are now ready to look westward!