I'm currently in a campaign, laying siege to a castle. Problem is, my cats range says I can hit his walls but it wont let me attack it, the gray ranged outline covers the walls but its like they are one tile outside of the range? Known bug? Apologies if this has already been discussed and I've been to lazy to scroll down the page further...

You need to have a troop within eyeshot of your target. Move something expendable up closer and you should be good to go.