We should really be able to backtrack on our spirit beasts. Most people probally tear through the game feeding it whatever stones are the most plentiful, not knowing what it would do. At the end of the game it makes a big difference. At the least, tell people what your beasts will mutate into.
I was actually suprised, this was a great TD game! I expected to be dissapointed by this, but it was really good! Good job on setting yourself apart from other TD games.