Which towers did you use? I can't seem to get past 49.

I'm starting by making a maze like Bitt said, using nothing but Pellet Towers. Then I upgrade and sell until I get to the following setup:



I have two pellet towers maxed out in the middle of the map, so they can reach anywhere. Then I have freeze towers second-to-last in every row. The last pellet tower in every row is the first to get upgraded, because I figure it'll be more often in range. Then towards the end I evenly place the swarm towers in the path of the flying creeps, which seems to take care of them. Then there's my last line of defense using some squirt towers.

What was your strategy?