This game is TOUGH!
I'm usually pretty damn good at these defense type games, but before you fixed that "Not a number" error, I couldn't even beat level 5.
Once you fixed it (or at least it looks like version 1.1 has fixed it for me), I was able to beat level 5 and get to about level 70 on level 6 before dying. I think I can go a little further next time... what killed me is those level 48 guys are RIDICULOUS - they have like 8 different things that they morph into each time they're killed, and each time they have 40,000+ health points. I'm guessing maybe you put an extra zero on their health? Or maybe you just wanted people to have to lose a couple of lives there. I actually managed to eliminate them in hopes of some big cash prize for doing it - I basically had to wait until they were almost at the end of my maze, then open up a hole somewhere else and plug the hole up they were about to walk through... so that they had to walk halfway back through the maze.
I must have done that at least 7-8 times before I killed the last version of their character, but the only thing that paid me was killing the first version.
By the way... is that a bug? It seems all aliens give me cash for killing their initial 'version' but not for killing the ones that they split out into. I think you should get paid for that as well. It might at least make the game more possible to beat?
My tips for those having a hard time:
Build your entire maze of vulcan cannons at first. Then go in and place a freeze tower and the tower that stuns enemies by 'slamming' the ground at every turning point. Put the 'slam' tower at the very end so that it gets 3 sides exposed to the enemies, and the freeze tower right before it, so that it slows the enemies down as they walk past the 'slam' towers. Once you get the freeze tower and the 'slamming' tower upgraded all the way, you'll get quite a crowd of enemies that 'build up' at each corner, stunned and slowed. This is a great point to attack with SAM towers that do splash damage (by the way, a SAM tower is surface to air missle... that's another bug in this game. SAMs should attack air enemies only).
Place your plasma/BFG guns further down the maze so that they can pick off the enemies once their health has been dramatically reduced by the SAMs. Putting them in the middle is a great idea, too, since they will help you out quite a bit with air attacks, too.
I basically had my maze almost impenetrable by ground units, but air units are what made me lose. Make sure you have a couple of frost towers in the air enemies' path (they will attack air, too). This makes all the difference in the world on what would otherwise end up losing you a bunch of lives.
Make your maze as efficient as possible by creating a square that first spirals around the outside counter clockwise, and then at the bottom left corner starts turning up and back down. If you do it right you should only have a 1-space wide area for them to walk in almost every area except for a couple of turns toward the end of the maze.
Design the maze to allow you to create an alternate hole in the maze at the halfway point, and plug up the end. Of course this does you no good if you have enemies at the halfway point too, as they'll just walk right through, but many times I found myself trying to kill off some really hard enemies that were about to escape and no enemies were anywhere near my halfway point. Saved me a few lives and made me some extra cash.
My next idea is to try creating an entire row of maxed out 'slamming' towers with the theory that at 20% per 'slam', and towers on both sides of the enemy, it will be nearly impossible for them to move anywhere.
