Green (push-type) cards have high health, low attack, armor to reduce damage, and will push back offensive units. Place them in front of an attack to protect your gunners. After awhile, place them in the rear of your lane to prepare another offensive wave. Use push-type cards to defend low-priority lanes. Using a single high health push-type card in non-essential lanes can slow your enemy's progress long enough to allow your main lane to win.