This might just be my personal favourite tower defence game. I find this game to be a good stress reliever. This game does get challenging later on, but luckily, there are guides on YouTube that you can follow for many of the later stages should you get stuck. Although it's not a huge problem for me personally since I'm not really an achievement hunter, heads up to achievement hunters, some of the achievements seem to be bugged ("Endangered Species" and "Take a Day Off, Doc" in particular (Just a quick update, I now seem to be getting more aliens in my collection. For the longest time, I seemed to be stuck at 87 aliens but I've been able to get more, it just takes a while. I might just be able to get the "Endangered Species" achievement after all). I would especially recommend getting this game on sale.