A small useful thing I've discovered in my seleucid campaign (pretty consistent since it happened 3 times out of 3):

If you engage thde Parthian army outside Asaak in turn 1, they won't go for Asaak on their turn(even if it's almost devoid of troops, once even had it as low as 20/160 spearmen and 40/160 archers on huge after the battle) but rahter attack and take Zadrakata, which is pretty much undefendable at this stage(not enough time to get any troops there from neighbouring cities.

The good thing about parthia taking Zadrakata is that they will soon get a common border with Hayasadan(sp?) which tends to lower the frequency of attacks on your possesion.

The bad thing is that the common border with Parthia extends quite a lot into areas you wouldn't normally have to defend (I've suffered the occasional attack on Ekbaktana).