well. seems as you have just started this project right?
you should change the spawn system, you are creating a bunch of units at the same spot instead of spawning all of them at random points.
also give the Ghost(visible) ability to the creeps to make them able to run through other units.
also it could be very useful for you to read this turtorial: Link
it explains very well what memory leaks are and how to remove them. and your game got some of them.
Whell you know what is a problemm if I'l make the creeps gost visible? Creeps passing the trees they will enter on ich other. They will loocks like one creep and when they attacked the tower it blows by one hit.