I hate saying burn the Est Empires capital completely because you know, innocent people and whatnot, but god damnit burn their castle straight to ashes and for good measure destroy any other noble-esque mansions, grain storehouses, and guard barracks as you can cause fuck that country.
Then when you get back to the kingdom, burn that castle down too. Kill the shitty prince, the whore who started this, and the dipshit King who'd rather spoil his fuckwad son than respect his Duke.
Geneva Convention is nonexistent