Built a zigzag series of simple machine gun towers and then after
you had zig-zagged a few times, built bomb and rocket-launcher 
towers  because they have a good AOE effect which is needed in the
later levels. I upgraded 3 rocket launchers to the level where they
had 200 damage(the last level is not worth it) and one or two 
grenade towers to the maximum level and that carried me through the
fifty levels. Just concentrate on the AOE and you should be fine.