Shockingly solid!

I was skeptical at first; I didn't think that there could be fine enough control over the boulders to realistically stop the zombies. That may still be true at a higher level of play, but I was surprised by how in control I felt, after wrapping my head around it. Highlighting affected boulders was one of those little touches that went a LONG way towards usability, and it was only infrequently I felt like I suffered for something outside of my control. Full marks, you.