Basically, you have a gentleman's agreement with the creeps. They can't attack your towers, but you can't completely block their passage through.
I have to say, finding a good arrangement for my towers was a challenge. At first, I tried to build up the pellet guns as far as possible, figuring that was a cheap way to go. But the pellet guns, despite their phenomenal range once upgraded, were just too slow, even when I built up a maze that had the creeps on screen for a long time. Eventually, the maze would get overwhelmed with too many creeps on screen. So, a flyer would come on, while I was still trying to get rid of the creeps on the ground. And I would lose on level 47 or 48 (there are 49).
Instead, I came up with this '2' maze, with the squirt towers built up as much as possible in the middle. By getting the creeps to circle around the center, I'm able to keep up the maximum amount of firepower on them for just long enough.
Feel free to let me know about your best strategy for defeating the creeps.
Play Desktop Tower Defense at handdrawngames.
Other Tower Defense games:
Flash Element Tower Defense - TD
Tower Defence
Imminent
Update: I was able to kill all on normal level using a circle defense: See the map That's a swarm tower surrounded by squirt towers. Frost towers line the circle to slow them down. Score 6352.