Wow! After some 100 various layout trials I finaly found one that worked. Build the upper triangle first (until around wave 15 or so). Block tower maxed. Missiles to level 6. And then build the rest - and be prepared to build as fast as you can (including updating all of them) in the last 5 waves. Geo Defense from Critical Though Games is a fantastic game. (Not an ad. I just like the game.)