Caravel

Quote re Trebizond (which initially included Sinope): "A high range of mountains separate this coastal strip from the Anatolian tableland and this fact helped in maintaining the area under the rule of the Byzantine Empire, even when most of western and central Anatolia had been conquered by the Seljuks and other invaders coming from the east."

Crudely, Anatolia has the Pontic mountain chain running west-east in the north, and the Taurus west-east in the south (swinging north-east at the edge of lesser armenia), with a high tableland in-between, increasingly hilly as you go east. So the effect we want is of a natural division into 2 coastal strips and an interior, with lesser Armenia pretty much encirled by mountains apart from outlets to Antalya and Antioch. You might therefore have
Trebizond to Rum, Armenia mountain
Trebizond to Georgia hilly
Trebizond to Sinope flat
Sinope to Rum, Nicaea hilly
Antalya to Konya hilly
Konya to Lesser Armenia mountain
Rum to Lesser Armenia mountain
Antalya to Lesser Armenia flat

I have to say I would make the vast majority of borders "no river" - how many battles in history actually consisted of one army trying to force its way across a bridge (not a river) in the immediate presence of the main body of the enemy? Bridge battles to me are one of the least realistic features of MTW, but luckily it is easy to control their frequency.

Hope that's helpful and you're not looking for something different.