During the 90s Bungie released a Sci-fi shooter named Marathon. Pitched as the Mac's answer to Doom, the game is a fast paced first person shooter set on the ship Marathon after it's been overrun by aliens.
While it may sound simple the game is actually very clever and is far more than just a Doom clone. Gameplay tweaks like being able to aim up and down as well as sideways make it seem a bit more thought out and the atmosphere Bungie create and the story the game tells was rather unique for the time. As you explore the ship you come across terminals, the ship's AI uses these to give you objectives and communicate the story. The soundtrack and level design is done so as to go hand in hand with this and create a world that is bleak and desolate but enthralling because of it.
The ship is effectively a ghost ship, one overrun by vicious and powerful aliens and it's up to you to sort things out. The gameplay consists largely of the tropes most 90s shooters consisted of: running and gunning, strafing and figuring out how to open doors. The thing is it sounds derivative and dull but never feels it, there's enough of an atmosphere to keep you on your toes and make the game seem fresh and exciting.
Graphically it isn't up to much and yet the art style is such that this doesn't matter. Things are supposed to be bleak and basic so the graphics, even though they're awful by today's standards, do their job of bringing the world to life. The soundtrack is the same, it is sparse but foreboding and that works perfectly. Everything about this game is designed to drip with atmosphere, fear and violence. You're a badass space marine type but you still feel vulnerable.
Controls wise it works pretty well on iPhone, they can be customised slightly and I would always recommend fiddling with this but by and large they work well. These older shooters seem to translate very well to touchscreen devices when it comes to the controls.
Ultimately Marathon is unique, an early 90s shooter that was about more than the shooting. It's not perfect, Doom is the better shooter, but it is amazing. It's also totally free on the App Store so you've got no excuse not to try it. Now off you pop.