You know those cyclops that you get in the very beginning? Use those to defeat the towers. I realize that it is nearly impossible otherwise to defeat them, so make sure you keep atleast one!
To stop the enemy from spawning at both bases you must be far enough in to where they spawn from.
Going for the South base might be a good idea anyways, as it is smaller and therefore has slightly less defenders.
EDIT: Whoops, I see why that wouldn't work, I made a mistake with the triggers... download again and it should work. Sorry!
