Hint 1: Stack these auras with the others. Make sure they're back far enough to at least overlap the buff beams. Ideally you want these to overlap all of your towers to protect against spiders; this is less important if you're using a summoner.
Hint 2: Place a mage and three archers at each choke point. Try to get as many as possible on the buff beam.
Hint 3: Place four archer minions around each of the crystals to help protect from wyverns and spiders.