I got this game for a cent in a Humble Bundle, but it is easily worth full price.
This game is a "bullet hell" style shoot 'em up set in a creative futuristic steampunk timeline where the British Empire have colonized Mars. It's relatively simple, with only three actions (fire, special, and 'vaunt', which defends you from enemy bullets for a temporary period) but provides tons of depth and difficulty. There are five campaigns and a subset of ships for you to choose your own personal play styles with. While five maps may seem very short, the five difficulty modes will keep you coming back for more; the more chaotic the action, the more fun the game really is. Add many secrets and ship types to unlock and you have a decently fleshed out game.
One of the game's strong points is presentation. The pixel art is absolutely gorgeous. We're in an era where every single indie developer can churn out poorly made pixel art in an attempt to be "retro", and it's seem like we've forgotten how good pixel art can be. The only downside is that it make the action seem confusing, with enemy bullets sometimes being indistinguishable from the background. But in addition, you have the soundtrack, which is one of the greatest video game soundtracks I have ever heard for a game of this caliber. It pumps you up for the action and contrasts nicely to the crunchy explosions you'll hear.
The best thing about this game, however, is the local co-op. Supporting up to four players, the game can be played with a mouse, keyboard or controller, and is absolute tons of fun. In co-op play, greater strategy is required through the use of collecting one-ups, but in exchange it allows for greater firepower in the hectic stages. Combine this with a high difficulty and you have some of the most fun you'll ever experience in a shoot em up game, ever. I strongly recommend this for fans of shoot 'em games, especially bullet hell shooters such as Touhou
, but everybody needs to give this game a try.