Great concept!
The game's a lot of fun, not much more to be said. The graphics are plain as hell, but I can't blame you for that. Check out Speck Oppression. If you put some trippy little particle effects/lighting halos/glows in there, the souls will look more "soully", and this game could be a HUGE hit. It's part of the formula, though: the gameplay's fun, but it's not Bloons fun. So if you want a hit, you've got to upgrade from the Bloons graphics. If you're ever looking for a design partner, let me know.