Alien Invasion

Aliens are trying to invade earth and you are the last line of defense. You will need skill and good reflexes in order to survive, but maybe doom is inevitable?

Can you set a high score against the relentless invaders?

Alien Invasion is a classic arcade style shoot-em up with an increasing difficulty curve.

For the best result, play the game on a real Game Boy. It gives a better feel mashing those buttons and the sound is much cleaner than when you are using an emulator.

Physical copies will be sold at ferrantecrafts.com.

Source code can be found here.

Keyboard Controls

Move Left    = Left Arrow OR A

Move Right = Right Arrow or D

Shoot              = Alt OR Z OR J

Start                = Enter

Why was this game made?

The reason for making this game is that I had been learning how to program for the gameboy for a little while, and I wanted to challenge myself to see if I could push the hardware a little bit.

One of the limitations is that you can only have 40 sprites (8x8 or 8x16 pixels) on screen at any time. One game which I thought about would have an issue with this is Space Invaders (which this game is obviously heavily influenced by), because you already want around 40 enemies on screen, otherwise it would be a bit too easy/boring.

My approach to handling enough enemies is by using 8x16 sprites, where you will have two enemies on top of each other (each using for example 8x7 pixels, to leave a 1 pixel gap between them). An extra benefit of this approach is that it is also possible to make larger enemies, which would be destroyed in 2 hits.

Some extra changes I added are the possibility to add enemies of different sizes, for example you could add a thin enemy of maybe only 2 pixels wide. The same holds for bullets, so they can also have varying sizes. All of this comes at the cost of a lot of cpu usage, but figuring out how to keep it within the limits has been part of the challenge.

All of this hopefully makes for some engaging and interesting additions to an classic game format.

Another limitation I had in mind was limiting myself to a game which would fit in a 32KB ROM, without anything else (so not RTC, save RAM, etc.). One of the reasons for this is that I have been able to make physical cartridges which can play those ROMs of 32KB, so I could make my game as a physical distribution from scratch.