X = fire main weapon. This can destroy both invaders and their bombs and mothership.
Z = fire particle beam, which takes time to charge up in top-left. This is ultra destructive but can only destroy invaders; not their bombs or mothership.
Arrow left or right to control ship
Invaders that dive-bomb are worth more.
A mothership will occasionally appear and respawn invaders. Shooting it down is worth max points.
When the mothership spawns new invaders, they take a few seconds to materialize; shooting them in that state is worth more.
As you progress through the levels, the mayhem increases and gets faster.
Of programming note: the Animations and Sprt functions implement a general-purpose system for animating sprites in a declarative way. Just specify the state transition in a table and call the Animation function with the object to animate, and it will automatically transition the object through the specified states, including looping and chaining.
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