plexsoup already pointed out the main knots of this game.
Anyway, it has some solid mechanics to become a great tower defense game.
What I really like about this projects is the graphic component: hand-drawn games are something I really appreciate, and even though nowadays everything is digital, I think that drawing on paper with pencils what you have in mind and animating it in a videogame is -first of all, not a simple work- a unique and original idea to gain the interest of the player.