i think there should be an option for the attacker, too.
Defenders have their option to "retreat" in case of pillage or field battle.
But dont u think attacker has too little option?
what i want to suggest is to have an option for attacker, too. And i'd say this is more realistic way.
I think attacker should have an option to go on to fortress siege if the defender's "retreat" option is enabled. (you do this when you send the army)
So field battle1 (return to home), field battle2 (go on to fortress siege "only if" the defender's "retreat mode is enabled")
I think both attacker and defender has fair options then..
oh and another thing,, i think losing all the siege weapons if we lose the fortress siege is too harsh, and it is too discouraging for a war-based game. Im not sure if this is discussed or not, but i think it is too harsh to do that. Perhaps it is fair make the attacker lose 50% of the siege weapon, and defender gets that 50% of the siege weapon? And it sounds quite realistic.
Your option doesn't add much... After all, why should I care about the retreat status of the army? In sieges it will fight in any case...