[ ] Find the ringleader!
You need to find whoever's behind this and put a stop to them before they can get away. It won't be an easy fight and will almost certainly leave your city open to further damages, which you'll have to deal with later.
I really, really, really do not want to leave Kat around after he is shown murderous tendencies towards our citizens and allies.
Now, there are counter arguments. Cat seems to be planning on leaving Danville. He is of limited danger without the support of his fleet. If we screw up attacking him, he may refocus his efforts on destroying us. And of course, there's all the damages that were done and will be done.
To those arguments, I say this: We are probably the only king outside of bellwether who is in any way equipped to see animals as a threat. Kat's fleet was completely uninvolved in his attack on Danville and he managed more than enough damage without it. He is currently a nemesis, and I don't think that'll go away if he gets away after being thwarted. And while the damages that the cats are doing to Danville now are terrible, I would rather head off any future damages by Kat, Since he knows all of our defenses and seems to be able to infiltrate without any problem.
In short, let's finish him.