Some observations I’ve made whilst playing the game, in no particular order.
The attract sequence consists of three screens. Screen one is a list of all 6 enemies (they appear on-screen in a reverse explosion animation) and the control options. Screen two shows some simple instructions. Screen three is the high-score table, that uses the enemy explosion animations as fireworks.
Landers (main enemy) locate a human, make their way over to it, pick it up, and ascend to the top of the screen. They fire at the player occasionally. Once at the top of the screen they turn into “mutants”. They don’t seem to choose the closest human, and I don’t think they’re allocated a particular human to pick up when they’re created. They must choose one at random (source code will confirm).
Mutants make their way straight towards the player (jittering a little - must add a random amount to the y pos) and shoot regularly. They try to position themselves above the player (and can move off the top of the screen) in order to avoid being shot.
Bombers (small squares) move in a sine wave (wrapping around at the top/bottom of the screen) dropping small bombs. Bombs stay on screen for a second or two.
Pods appear (not sure) to be given a velocity when they are created, and they then don’t change their movement during the duration of a game. Shooting a pod results in it splitting into swarmers. Not sure of the behaviour of swarmers.
Baiters are UFOs that have erratic flight patterns. They seem to alternate between hovering and darting for the player. They shoot at the player frequently, appear at intervals during the game, and don’t have to be destroyed to finish a level.
Destroying all humans results in the landscape exploding and all remaining enemies becoming mutants.
There’s a starfield behind the mountains - I’d forgotten about that.
The game uses palette rotation all over the place instead of animation. I’ll have to write a palette rotation routine. In fact, it’s probably single palette indexes having their colours changed.
Catching a falling human after destroying the lander trying to carry it away gives a bonus of 500 points, and landing it safely again gives another 500 points. Allowing it to fall gives a bonus of 250 points.
When destroyed, the player’s ship turns white and flashes on and off a few times before exploding.
There can be a maximum of 4 bullets shot by the player’s ship on-screen at once.