My favorite game so far! A nice concept, good graphics, progression of mechanics, cool tune - it's got everything. The only thing is that I'd make it impossible to start your turn without summoning a single creature. This happened to me on the first try, and I watched empty spaces try to attack the hero one by one.
Maybe some kind of a failsafe? If there are no creatures, then it's impossible to press start and a message appears. Could be helpful.