The idea is that you can push enemies around repeatedly as well as stand in front of them in order to slow their advance, if you get between an enemy and the last column of the map, you can push them all the way to the front, and then in front of any tower you still have alive. In theory, if the player is fast enough, the game can be beaten with just 1 or 2 turrets, because there is no limit on how much enemies can be stalled by pushing.