The unit going to attack the castle is the critter. AFAIK, there's only 1 unit involved in this, and that's the unit attacking the critter. The "castle" can be terrain. All it has to have is a location to know where the "castle" is. And just so you know, detecting a right click only detects that you right clicked, not WHERE you right clicked. There's no easy way to solve the problem, and obviously critters aren't ideal.
The only problems I forsee is: dificulty getting any unit to return to "attack mode" after not being thrown far enough, changing unit speed, and the cf which would develop due to the massive numbers of critters. It's also pretty ugly and slow (so use a shorter running distance). One recommendation, though, to improve the "look" of it. Turn off vision for the player, and use a shared vision map revealer to show where you want to see (like around the castle). The critters will come, but they will appear out of nowhere, while you still own them, and they will disappear when you fling them high into the air.
You could use zerglings to track each and every critter you make, but it'd take some serious triggering, and would be difficult, however it would have the side effect of slowing down the unit at a controllable degree. You could randomize the size of the locations centered over the critters, anywhere from 1 pixel to an inverted location. The critter isn't fast enough to outrun a 1 px location. Also, you should have the units moving towards the left, not the right, just to avoid any possible collisions switching the units.
If what I'm saying here doesn't make sense, that's because it shouldn't. I'm not going to explain in detail how to do this, just that it's theoretically possible to do. I can't give you the triggers, since I'd essentially have to write the map for you then. If you can't figure this out on your own, it's not worth the trouble to do, since it really takes a long time to understand the process. You can read up on things like inverted locations and slowing down units in the wiki/search.
If you wanted to use kakarus, you could stack units, but unfortunately, there's no way of keeping such a unit slowed down, since slowing down units relies on there only being one unit in that spot. Not that it matters much, since all the units are the same.
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