Huge Loophole with Invaders

I was thinking about them today, trying to realize just what was bugging me about them, when I figured it out. As proposed, there is no way to attack an Invader, once it moves onto a square containing a Wall, except with another Invader. That means it is impossible to properly defend against them, except with Invaders. Unfortunately, I didn't figure it out sooner so I could point it out, hopefully this isn't too late.