How close to a realistic Seleukid army would this be?

[Zradha Pahlavan]

Ha ha! Hide behind your walls, Greek! You won't stop the might of the Pahlava otherwise, and even then you're only wasting everyone's time! You should just surrender, then maybe the Great King will be merciful to you...

Other things:
Stone walls won't help much against sapping. The lousy Saka used that trick against me once, and broke through the light infantry that were supposed to hold the line there, resulting in a bizarre battle in which opposing sides of horse archers ran throughout the city shooting at each other, all the while trying to elude each other's lancers.
But if you have a well-armored phalanx group, you can plug any gap in the defense. It's good to have a few of them around, just in case.
Another tactic is to lunge at the nearest steppe town, take it, and hold it long enough to get some regional steppe horsemen. Then you can fight fire with fire.