I'd recommend Kingdom rush and E.R.A, both very challenging and I like the latter one even better!

I gotta say ERA has kinda redefined the tower defense game with the concept of hero and skills, anyone here playing the game, might be good if we could talk about some strategies and tricks.Haven't been so excited since the Kingdom Rush release.

Fieldrunners2 HD is out, and it crashes like crazy!!! Just saw the thread in TA and they're saying you need to turn off iCloud to get it working properly.

Anyone is playing it too?

I love play Clash of Clans! It's really fun!

I like Tiny Heroes, it's really fun to play!)

if you are a romance of three kingdoms fan, try 3 kingdom TD in hd quality for ipad and its FREE....its so much fun and there's bit fiction history too just like the storyline in the dynasty warriors series.....(the red cliff battle is da best!!!)

This looks awesome, also E.R.A seem to have a FREE version now: E.R.A TD FREE for iPad on the iTunes App Store

I definitely suggest GemCraft. It's fairly simple, but I absolutely love the combinatory mechanics, and a decent length. Kept me entertained for weeks.