VoxeLibre

Im not sure how that works. Is it the structure or simply the place. Meaby they are spawning in that place no metter if you destroy structure or not. It would be interesting experiment to see, but meaby its easier to simply get answer from someone that knows the source code of the game. And also since its open source i understand that anyone could teoreticly search for that part in the code and figure out how spawning works.

But if you want to stop them there is one thing that comes to mind. Mobs cant spawn on slabs or carpet, basicly anything that is not full block.. So if you showel the grass they also wont be able to spawn on that. I would just place slabs around it. You can make it look nice if you want.

So i gues ten blocks in any direction of the tower would be enough, meaby more, idk. But thats what i would do.