How to write a game on a ZX81

I made a video demonstrating live coding on a 37-year old ZX81: I write a playable but terrible game in BASIC, on a machine with 1kB of memory.

In fact, since I made this I’ve spotted several ways in which I can make the program both smaller and faster, and I notice that I completely forgot to explain how the various extended shift key modes to get graphics characters work, and I think I neglected to mention that the machine doesn’t use ASCII and can’t display lower case, and…

But enjoy.

(Incidentally, the secret is LET C=C-(INKEY$="Q" AND C>0).)