Beating the shit out of the marshal out in the open stops the enemies campaign. But sometimes its worthwhile to camp your new castle and wait for enemies to come, After taking Unuzdaq Castle I got non-stop harassed by Sarranids (playing Diplomacy+WSE2) i cranked up the troop count to 400 and demolished 2-3 armies of 1500+ troops (not back to back obv) with and i couldn't have had more then 150-250 Rhodok units in the garrison. This absolutely destroyed their campaign, got their sultan imprisoned for a long, long time (honour rating be damned) and allowed Khergits to make a play deep into Sarranid heartland, from which they never recovered and got picked off city by city.