I played a siege back a year or so for a remake of the Albion campaign (so I'm not sure if the rules are the same) and for attacking I was allowed to give each Krox a ladder since they counted as a rank-and-file unit instead of monsters by our ref's decision. So I armed myself with a JSoD and krox to storm the walls then blasted any armored men from the walls with sallies since they can unleash so much hell in one round.
A far as defending goes, I'd have to know how many points and what equipment you could take (boiling oil, rocks to throw, etc.)