I've been working on a zombie castle defense game that you might be interested in. http://forums.toucharcade.com/showthread.php?t=103053 It's almost ready to be submitted so keep an eye on it (promo codes will be available after release) In the meantime I suggest you check out Zombiesmash.
Zombie Attack! Second Wave from IUGO might be something you'd like, it's tower defense, but it's fun. There was this other one that was actually a castle defense game, but I can't remember. I'll post it if I remember.
These zombie games have some tower defense aspects... Lamebo Zombie Wonderland Pro Zombie Soccer Zombie Pie