XCullX: the purpose of the spawns is that they DO pass over the towers, it makes it harder.
You just need to double up the corners, it costs more but it is possible
Finally! got the hard badge...
I might try to get to the top lvl but that is it, I am NOT going to go for the "Impossible" badge... I dide almost 130 times on lvl 30...
 "Whiteboard Tower Defense"){kind=icon}
 "The Tickler"){kind=icon}
 "Robot Dinosaurs That Shoot Beams When They Roar"){kind=icon}
 "Desktop TD Pro"){kind=icon}
 "MoneySeize"){kind=icon}
 "(+) Argent Burst"){kind=icon}
 "Gamma Bros"){kind=tracking}
 "rotaZion"){kind=icon}