Think of the hardest, most frustrating Android game you've played thus far. Is it Angry Birds, with its unparalleled addictiveness? Or how about Plants vs. Zombies, which has a seemingly infinite number of levels and is within spitting distance of Angry Birds' can't-put-it-down factor?
Or - dare I say it - perhaps none of the games you've downloaded from the Android Market have been difficult enough for you. Perhaps you're on the lookout for something a tad more complex - a tad more like this:
Indeed, if a vexing mobile game is what you want, you couldn't do much better than Orbital Defender.