Alien Invasion

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Overview

Alien Invasion is a Space Invaders style shooter for the Game Boy, made as a challenge to see how far the hardware could be pushed. The Game Boy can only show 40 sprites at a time, which is a problem for a game like Space Invaders, where you want around 40 enemies on screen or it gets too easy and boring.

The solution was to use 8x16 sprites with two enemies stacked on top of each other in each one, with a one pixel gap between them. That also makes it possible to have larger enemies that take two hits to destroy, enemies of different sizes such as very thin ones, and bullets of varying sizes. All of this costs a lot of CPU time, and keeping it within the limits was part of the challenge.

The game was also built to fit in a plain 32KB ROM with no extra hardware, so it could be put on homemade physical cartridges. It was entered in the GB Compo 2021 game jam.

Made by: FerrantePescara
Released: 2021

Controls guide

Playing

Button and key	Action
Left/Right	Move your ship along the bottom of the screen
A`X`	Fire (one shot on screen at a time)

Title and game over

Button and key	Action
Start`Enter`	Start a game from the title screen
Start`Enter`	Return to the title from the game over screen

Cheatsheet

Points per hit

50: Plain aliens of the first waves; slowest to fire again
100: Aliens that reload a little faster
125: Narrow aliens with a quick reload
150: Aliens that fire the wide bullet
200: Common in the third and fourth waves
500: Rare aliens in waves five and six; can fire again almost at once

Rules

Lives: One; any enemy bullet ends the game
Invasion: The game also ends when the aliens reach your row
Slots: Each slot holds two stacked aliens or one large one; every hit scores
Waves: Six formations that repeat in order
Speed-up: Each cleared wave makes the aliens step faster and shoot more often
High score: Shown on the game over screen with your score

Tips

Your shot and an enemy bullet destroy each other when they touch, so firing straight up at a bullet coming down is a valid way to clear it.
Only the lowest surviving alien in each column can shoot. Wiping out an entire column removes all fire from that lane and gives you a safe spot to stand.
You can only have one shot in the air, and it comes back the moment it hits something. Aiming at low aliens instead of far-off ones keeps your fire rate up.
The game ends as soon as the lowest aliens reach your height, so in later waves, when the formation steps quickly, keep the bottom row thin.
The 500-point aliens in the fifth and sixth formations have almost no reload time. Take them out first, both for the score and to cut down the bullets on screen.