...is counter-siege heavy weaponry. You know, e.g. trebuchét-ish thingies firing their loads onto the unfortunate besieger. Obviously, they should only be available on the larger types of fortifications, and have limited ammo, like any other missile _unit_ in the game (the unlimited ammo of the wall towers in city sieges have, from time to time, caused me much grief).
Another suggestion would be to make some "plateaus", on the larger types of fortification, built on extended sections of the wall, where it would be possible to place one's faithful ballista unit, catapult unit or whatever one may have. The risk with this would of course be that enemy artillery just as easily could destroy yours. There could for example be room for 2-3 such "plateaus" on each lenght of the wall in larger cities.
Whaddya think?
Maybe too late to implement such things in EB, but anyway.
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