As Durk says, your only real option is to wait until the defenders starve and surrender.
Siege towers or RGD cards can make a breach from time to time but it's hard to get two or more before the defenders repair them, so usually It doesn't help much.
And bear in mind that if the city has an unblockaded port they will never starve.
So if you play as a pure land power like the Parthians, just forget about trying to take coastal cities with level 2 or higher walls. It will never happen!
Being barbarians is hard!