I enjoyed the game play, music, and graphics.
The wall mechanic was a new one to me for this genre and was the only reason i kept playing. The ability the change formations added to the fun, however I found my self only using two of them.
I read your comment below about upgrades and have to disagree with you, upgrades would have added allot of substance to this game; a change the formations upon upgrade would have added allot of extra joy, or projectile
strength to deal with with increasing wall structure.