geoFighter is the closest in design and simplicity. I would recommend Isotope though, because of it's non-simplicity... this means: Loads of levels... really, multiple Worlds containing Multiple Planets, each planet equals one level... And the levels are not just rectangles... they are mazes and some pretty uniquely designed paths. You can use the walls to hide and avoid bullets, but be careful, some bullets and enemies can go through walls. Upgradeable ships... you can buy tons of different upgrades... there are three classes of upgrades: Abilities (these add active things to your gameplay, for example you can use the "Bomb" ability to kill/knockback all enemies near you), Ship Upgrades (Add more life, more credit gain etc) and satellites (only one at a time allowed... follows you and does different things... there's one healer, some shoot in random direcions/nearest enemy etc, some frequently bomb a small area around you...) Enemies... LOTS of enemies... turrets, worms, and some other totally interesting enemy types. There are (as far as I remember) 3 boss types, big worm, big laser (annoying... you need to hide behind a wall to dodge the laser sometimes... then wait until he has to reload and quickly smash him a bit... back to the wall...) and big enemy ship (this ship is funny... comes after you slowly and fires lots of bullets ). There are also two BIG bosses (they got their own levels... starting the levels will call the boss, these levels are rectangles and no mazes...)... one of them is a turret (looks funny when you see it before it fires... it looks so harmless... then it starts spamming THE WHOLE LEVEL with bullets... whoa and loads of Arrow type enemies spawn from it's position, making the boss even harder...) and one is a big wheel (is has got 2 modes: red and green. If you shoot it in red mode, it will not only take NO damage, but it will also fire bullets in all directions... green mode lets you attack it, but green mode doesn't last long... hurry! ) The replayability of Isotope is huge (I'm still playing it sometimes even after months... I just felt the urge to kill the Turret big boss... hm still enjoyable fight), I would recommend buying Isotope if you want a Retro-Style Twin Stick shooter
CIRCUIT_STRIKE.ONE is not quite a GeoWars like game, but it's a good 2D shooter nevertheless. Try it out.
There are none. It's the most under-developed subgenre of the appstore right after swedish napkin folding games.
