Space Tripper ($3.99) by True Axis is a top down arcade shoot 'em up (shmup) that will definitely keep you on your toes. Apparently, this same game appeared on many platforms before finally landing on iOS, as well.
Space Tripper comes fully equipped with great graphics and fluid, simple controls. Just tap the left side of the screen to switch from widespread fire to a more powerful, single stream of laser pew pew. Tap the right side of the screen to switch the direction of fire (right or left). Although this works fine, 360 controls would be fun to try in this game.
There are three ways to play the game — and three difficulty levels — Campaign mode, Score Attack, and Challenge mode. However, you must play Campaign mode to unlock the other two modes.
There's definitely a learning curve with this game. It took me a bit to get used to the top down gameplay with the tilt controls, but once I did I got very into it. You are constantly maneuvering around enemies, and avoiding enemy fire. Plus, one hit from an enemy (or an enemy bullet) and you die. That kind of puts a damper on things; I found myself in panic mode almost the entire time I was playing. It certainly takes some skill to get far in this game.
There are 14 levels total and four worlds to wreak havoc in. The fast-paced action combat, including many unique enemies and boss fights, is sure to keep you busy for quite some time. There is a lot of content to get through and the game has just enough challenge to keep you in a constant state of gaming alertness.
The only thing I'd consider changing is another option for moving. It's almost impossible to tilt-to-play when lying down, or even in certain sitting positions. It is a top down game, after all. Some sort of joystick control may come in handy; you know, for those times when you're too lazy to actually sit up and play on your iDevice.
Oh, and did I mention there is a "Cheat Mode?" You can choose unlimited lives and complete invulnerability.
From the app notes:
"In Campaign mode, go to the pause menu, and tap the top left corner, then bottom right corner, then top right corner, then bottom right corner. This will bring up the cheat menu. As soon as you enable a cheat, all leaderboards, achievements and progress saving will be disabled until game over, or quit."
Space Tripper is available in the App Store for $3.99. For a universal app, and given the great deal of challenging content, I think this is a great deal.