At the beginning of each new game you will get a lot of adjustable variables. These control how fast you reload, how quick the aliens move, how many of them there are, and how many aliens thick the advancing horde is. Once you're happy with those, you get to input what type of alien, square or triangle, makes up each row, from the top down. Then you get to actually play the game. :D Use the arrow keys to move left-right and the up arrow key to shoot.
This is made with 100% pen, which is surprisingly simple to do. If you need a load of stuff that moves, and would exceed the clone limit, pen may be for you! I mostly did this to see If I could do it, so it probably won't get many updates, and definitely isn't a beautiful, cloud-multiplayer-3-dimensional-2,000,000 line work of art. The script is heavily commented, if you want to take a look. Thanks to LVgaming for the laser-firing sound.