My 4 year old got me into this game. True story.
It's simple enough that my young 2 and 4 year old boys can get on it, have a lot of fun, and do ok. And it has enough strategy and challenges that I found myself quite drawn to the game. I've found myself quite looking forward to getting back to my game when I can. Admittedly my kids are quite iPad proficient, but you get the idea.
You basically control a little dude, a king or something, and lead a little cute army of knights and what-have-you along a road of traps, towers and enemies to destroy the enemy castle at the end of the road. A lot of the game play is automated, that is to say, you don't have to press a fight or attack button. If you just stand near an enemy you start chopping away at it.
It's a strange sort of game when it comes to the violence. You are chopping bad guys down with swords, but the proportions of all the characters give them a child like appeal. I think it's actually rated 9+. It's like Mike the Knight all grown up (if you have kids you might know who I'm referring to).
I'm pretty sure the game is free, and you can make in app purchases of "Gold" that helps you along, but I haven't needed to purchase anything. I think the game makers could easily have charges a few bucks for this game.
I've been playing it on my iPad mini and it's a perfect fit.