Rogue Legacy is what gaming needs more of. It's challenging, detailed, polished, and absolutely enthralling.
Modern triple A gaming has this habit of treating me like a stroke victim. The game mechanics are either something to keep the player from falling asleep between plot points (typically found in RPG's) or lacking concepts other than filling the closest enemy with absurd amounts of lead and hiding behind cover if the same should happen to you.
Rogue Legacy on the other hand is what would result if Dark Souls and Castlevania decided that S&M was too tame for them. It's a 2D platformer in the style of Castlevania where an overly determined family of adventurers delves into a cursed castle. Every time the character meets his unfortunate demise (and he will, a lot) you continue on as his plucky son or daughter. After the 50th character or so you have to wonder if this castle is entirely populated by this single lineage. The gold collected can be used to unlock more potential classes future generations can become, or upgrades to equipment and stats. The joy of Castlevania was beating the game with only Simon and his few different items, making it a feat of skill rather than grinding up xp and items to beat the game. Rogue Legacy manages to combine both aspects, character progression and a feeling of accomplishment through skill. Feeling like you are developing with the sprite on the screen makes for a satisfying sight when you beat a difficult section or boss. This is where Dark Souls' influence can be felt. Some sections require the player to be both quick and thoughtful, with a misstep on either aspect leading to a wallpaper tastefully decorated with what is left your character.
In short, it is a beautiful game well worth the asking price of $15. It is a great addition to the roguelike genre, and I hope it leads to more of the same. Being genuinely challenged is too rare an experience.