Good game, just a little short.
One thing though:
The healing is meaningless out of 2 reasons.
1. You can just summon a new defender into the same spot as the one that was killed before the monster can pass the spot. (Just as ultimatum479 already stated)
2. Healing costs 20 points and heals only 1/4 or 1/3 of a defender's health while summoning a brand new one with full health into the same spot only costs 35 points. So healing is more expensive than summoning a new one.