Assault Android Cactus is a very simple game that demonstrates just how important good execution is in game development.
This is a twin stick shooter with multiple characters that have two weapons (one upgrade-able spammed primary, another more powerful attack on short cooldown). The health-death mechanic is bit different because health loss costs you score/weapon level and you only really 'die' by running out of time limit.
The levels work mostly by changing the terrain on the fly rather than having you move towards something. So instead of more traditional 'scroll left' type of map, think of an arena where walls and grounds are changing every now and then.
The enemy variations are pretty good IMO. You got your standard slow dumb cannon fodders to bigger mooks with varying tactics (some shoots seekers at you, some ground pound for shockwave, etc.), swarmers to turrets to verticle missile launching robot dogs, it's got healthy variety in both design and function.
So all of this probably reads rather conservative in terms of mechanics and yes, you probably have seen all of these bits and pieces before but you probably haven't seen them to this degree of polish. Dare I say it, this game plays like indie take on twinstick shooter with the famous "Blizzard polish". Fans of the genre would be amiss to not have this in their library, and for new comers this game is really good introduction to the genre.