I tend to have less castles than normal with the Turks, as they can recruit some excellent units from cities (janissaries as well as earlier horse archers), but their castle units are not to be overlooked. Ottoman Infantry, Naffaratun, later cavalry, and (in the earlier period) Turkish Archers are all fine troops. The problem is finding a good spot to place your castles. In general, I agree with the "if it can build a port, make it a city" rationale, so try to place castles in landlocked provinces with decent growth and access to nearby cities in case of attack. Further, Baghdad can make a fine castle for defense against the Timurids and Mongols. This is vital - if the Mongols/Timurids come after you, you are going to need a multitude of well-defended castles on your eastern frontier.

I also like to keep the Byzantines kicking for as long as possible. Before gunpowder, it is not very beneficial to take them out, as you can't possibly fend off all the European powers that will want a piece of you once you get into Europe proper. Additionally, it's cheaper to use war fleets to block the two land bridges than to keep a full-fledged army nearby. This keeps out Byzantines AND crusaders quite effectively. They make a fine "buffer" between you and Europe this way, and you can turn your attention to Mongolian invasions and cleaning up the Egyptians at your leisure.