Have you room in your heart for one more touchscreen tower defence game? If not, well, could you try and make room? Radiant Defense is really pretty and really smart, see. It's a treat to play, and it doesn't even let the freemium model ruin all the fun.
You know the drill: wave after wave of creeps are spread across a series of maps, and you've got a range of different turrets to place in order to protect your bases. Radiant's turrets are great, but it will take you a while to try them all out. A handful of the more basic offerings are available from the start, while a second tier will require the placement of a costly research unit in order to unlock them. A third tier will only become available if you opt for some in-app purchases.
This isn't as bad as it seems, though. There's still a decent selection of gadgets to knock around with regardless of whether you pump any money in, and there are only a handful of IAP packs anyway, meaning that even if you do bust out the credit card, you're not going to be paying much more than a few pounds.