Get stronger with upgrades
What gives this platformer an extra addictive quality are the RPG elements that let you upgrade your hero. Gaining experience through killing monsters and finding special books, your character levels up, letting you choose to upgrade one stat for either Strength (for more damage), Dexterity (chance to critical hit), or Health (higher overall hit points). You'll need to choose your stats wisely to create a perfect fighting machine.
You also can upgrade your equipment. Using coins earned from killing monsters and destroying decorative items in each dungeon, you'll be able to buy new weapons, armor, rings, amulets, and potions. You often won't earn enough on a run to buy more than one or two items, so it's important to buy things that will make you survive the longest early on.
I really like the RPG elements because they give you a goal to work toward in an otherwise hack-and-slash type of game, but the system isn't perfect. Instead of giving you a choice of weapons, for example, you can only unlock one weapon at a time, so you'll have to unlock and use every weapon as they become available. This is true for armor, rings, and all other items you can buy. I think it would be better to offer some choice here so you could focus on being a swordsman, for example, instead of using whatever item comes next in the upgrade chain.
Dungeons and bosses
There are five differently themed worlds with 10 randomly generated dungeons in each. When you complete all the dungeons in a world, you have to face an extremely challenging boss character. At least, it will be challenging at first, but like many action platform games of the past, you can learn the behaviors of each specific boss to take advantage of his weaknesses. Killing a boss gives you a big chunk of experience points, so figuring out his moves is crucial to leveling up and making your fighter stronger for the next world.
Devious Dungeon comes from a long line of great 2D old-school platform action games on iOS from Ravenous Games, and it shows. The controls are precise, it's easy to jump in and start playing, and there's tons of content to explore. And with the addition of RPG elements in Devious Dungeon, it gives you an even better reason to keep playing and make your hero stronger.
The fact that each item unlocks one at a time is a bit of a letdown because you can't really specialize (or play a "role" as in role playing), which somewhat takes away from the overall enjoyment. But with random dungeons, increasingly difficult monsters and bosses, and secret areas to find in-between, Devious Dungeon is still makes for a lot of fun on your iOS device.