Seriously, it's not worth more than a Newgrounds flash game...
The "difficulty" of this "strategy" (lol) game simply comes from the fact that enemies pop up faster and faster as you progress along the "adventure", and from the highly unfriendly user interface.
The "lasting appeal" is due to the fact that in order to get decent features (like better units and extra slots) you need to spend hours doing always the same thing ("killing" zombies) to get game money to buy the extra features. To sum up, this boring as an RPG combined with Defend your castle. No-lifers will enjoy, I guess this explains all the 9s and 10s...