I've been a fan of Fieldrunners 2 for a while now -- it's my opinion that this is the best tower defense game ever made. With rock solid gameplay, plenty of content, and lots of little upgrades and tweaks to make, Fieldrunners 2 sits at the top of the huge pile of games that this genre has created. But the biggest drawback to Fieldrunners 2 at launch was that it wasn't natively compatible with the iPad, and this release, Fieldrunners 2 HD, that problem has been solved.
Unfortunately, the two versions for iOS are completely separate -- the app's not universal, so you'll have to rebuy the game if you want it on iPad, and there's not even iCloud integration, so your progress won't transfer over, either. That's kind of disappointing. It makes sense that Subatomic wants to make the title as profitable as it can, but it's too bad that the decision can hurt the company's most loyal customers. Otherwise, the game is great, and the HD graphics show off just how terrific all of the action looks. I don't believe there's any new content since the original iPhone release (though a new update since then has added a map and some other fixes), but there's so much here already that's not really a concern.
Fieldrunners 2 HD is available right now for $7.99. That may seem pricey when compared to other iOS releases (especially if you already purchased the iPhone version), but really, when you look at the quality and amount of content on offer here, especially if you're a tower defense fan with an iPad, picking this one up is a no-brainer.