bit Dungeon arrived on the App Store a little while ago, and if you missed it then, you'd do well to try it out now. Quite a few games have used the old 8-bit, top-down RPGs as a rubric, but bit Dungeon is probably the best example I've seen of all of them. You simply go from room to room, Legend of Zelda-style, hacking away at enemies, getting better gear and surviving as long as you can.
The controls are interesting -- rather than using virtual controls, you simply tap on the screen to move around, and then again to attack. This ends up working kind of like Diablo, actually, where you tap to move out of the way of certain incoming spells or monsters, and then tap back in for your own offense when the way is clear. The scheme works very well, and it allows you quite a bit of precision when you need it.
The game can be very harsh, in that when you die, that's essentially the end of that character. But I found health potions to be readily available, and as long as you make some smart choices while leveling up (and make sure you dodge as much as you can while in combat), the game is actually very rewarding and enjoyable. The great music and graphics help a lot as well. bit Dungeon is a really terrific title that's been a bit overlooked on the App Store. It's available for US$0.99, and you definitely shouldn't miss out.