Desktop Tower Defense

So, I've figured out a pretty good way to beat medium desktop tower defense but I don't think you can do it without letting any of them get by. The best I've done is 19 with 1 spaceship getting through and I've tried it probably 40 times since then; all of them being failures too. It's so frustrating to get to level 48 (Probably 20 minutes in?) And then 1 damn creep get's through...
Anyway, Here it is.

Here's my Desktop Tower Defense Strategy. I call it the modified spider.