It's hard to get more than one star on this, even with more than 50% of your buildings on the outside, your defenses will take care of you. The 3 springtraps on the East side of the map are placed there because 90%^ of the time, thats where the sninjaed WBs and giants end up. Putting all your Mortars in one space make it easy for them to cover each other and the areas around them. Keep in mind, this layout is most effective when your defenses are maxed out (obviously), so your experiences may vary.