Also, sneakdog, the enemies start out as timing issues, but the last levels really use them to full effect, forcing you to concoct completely different solutions.
Honestly surprised to find this game quite boring. Seems to add little to the inventiveness of other games, such as Bloons TD. I would suggest making what's happening clearer, slowing it down a bit, allowing you to click on an enemy to see its health, and on a tower to get a sense of what it can actually (whether it can target ghosts, for example).
First try: level 5;
Second try: level 9;
Third try: completed level 10.
Pretty similar to other games like this, eg. Traveller IQ. Few original features except the map changes.
Great games, could do with more dynamism (like more traps, walls, towers and mana options) but it's not hard, particularly if you hold down asdf and then click outside the game, then back in again.
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That would be too easy ;)