I initially misunderstood where the enemies would come from, and it ended up taking a very long time for anything interesting to happen as I waited for the monsters to cross the screen to my gnomes on the far right. The balancing of rocks seemed strange as well, as typically in a tower defense game you can buy a new tower after the first successful level, but I had nowhere near enough even for the cheapest gnome.
That said, once it got going, I had a nice little time!