If you're one of those people that just can't get enough of defense game you should check out the launch trailer for Keep off my Hill, the title just released for iPad from Isopod Labs. It looks nice and colourful, with plenty to do based on having to scroll through multiple sides of your base. It's also free, so as long as there's no overly aggresive in-app purchasing, it's easy to get a hold of as well. 

If you give it a go, tell us what you think in the comments.
[App Store Link]