They released an update for Kingdom Rush on the iPad about 1-2 weeks ago. This is still the best tower defense game I've ever played and always enjoy going back to it.
In the new update they released a hard mode and two new boards to play on. Glacial Heights and Ha'kraj Plateau. Neither of which are easy to beat and it has taken me this entire time to beat them.
I started out playing the new maps and finally got 1 star in each of them. I looked at the achievements and noticed one of the new ones was to beat the campaign on hard mode. Since it had been awhile since I've actually played it, I started working on that. I flew through hard mode. It almost seemed too easy. Was enjoyable going back through and beating the entire game again.
So then I went and started back to working on the new boards that released. I focused on Glacial Heights first then went to Ha'kraj Plateau. On both boards it took me using the gems in the game to buy a bomb that blows up everything on the screen to actually win them both. They are very hard to beat but it is so much fun to play.
If you don't have this game, it's one you should really considering buying. It's well worth the money.