Besieged
Besieged is a $1.99 castle defense game.  You control a group of archers defending the castle.  As you are attacked by skeletons, you use your archers to take them out.  There is a time lag between when you launch your arrow(s) and when they arrive though, so you have to time your shots based on the speed of the attackers.

While the game works, I found it a bit limited.  Other games in this price range (or even less, like Defender Chronicles on sale now for $0.99) offer a lot more in the way of defenses.  I hate to give bad reviews, but there is really no comparison between the depth of this game and something on the order of Defender Chronicles.