the sassanids infantery have a BIG problem

you can only make their medium infantery in the home territory and their spearman are, well... they are just spearman!
All you can do is use the horses and archers and use the spearman to block the attack.

yesterday I encountered a roman army with very good cavalry and I nearly lost. Normally I crush their cavalry while my spearman hold the line by piling their own bodies and hold till I am able to attack the roman infantery in the back after returning from the chase of the fleeing roman cavalry.

The only way to conquer a city is to wait it out, forget about fighting on the walls. They could have added one decent infantery unit...