We could also scrape off most of the trash with
Forceward and
Hand of the Faithful leaving us with things that are immune to magic or can make the save. If we get scrolls of HotF we could send in the cats to screw with their troop movements as we launch the attack.
Sure they could counterspell it easily, but if they pop up all over the place in the middle of a chaotic attack I wouldn't expect it to happen smoothly or efficiently. In terms of bottling up trash and sowing confusion across the fort I think it could do the job.
We could also try planting some
Imploding Stones on support pillars and similar locations to try and physically block motion on various areas as well, if we have time to buy them.
If we're going to do this in the future we should make some
Moldwretches with a brown mold link. We could arm them with sultan's tribute style munitions and let them start their very own ecological disaster.