It's tower defence. A kingdom of goodly witches is under siege by waves of
invaders from an evil kingdom. Deploy the witches as best
you can to defend the inner sanctum of their castle at all
costs!

To place a witch or upgrade a witch costs money. You get money for each invader killed
and at the end of every wave, you get 10% interest on any money you have left.
To win, you will have to judge when to save and when to spend.