Wayyyy back in the olden days, when Warcraft III was all the rage and World of Warcraft was just a glimmer in Blizzard’s eye (OK, not that long ago), I used to love playing Warcraft III custom tower defense games. Basically, packs of monsters would run down a path trying to get to your base. You would try to kill them all by building towers along the sides of the path. If too many got by, you lost. The more you killed, the more money you got, the better towers you could get. Of course the monsters would get tougher and tougher, and if you missed any you wouldn’t get the money for killing them, and it was a downward spiral from there. There’s tons of different towers that you can make, some towers are better against certain types of monsters while others slow down the monsters so your towers have a longer amount of time to kill them.
Pixel Junk Monsters is basically tower defense, but I don’t think that’s a bad thing. The music is pretty rad. And old old tower defense games didn’t have a crazy looking builder like this game, you just had a crappy peon.