I have finished porting my 3D ZX Spectrum game, Cosmic Payback, to the 16K ZX81. You control a spherical pod, bouncing through space and retrieving all of the flashing tiles (which represent gold).
Please find attached to this post some screenshots and, more importantly, the ZIP folder containing the tape in P format and the game manual. This game uses a pseudo hi-res technique, and is therefore capable of running on any stock ZX81, with any RAM pack of at least 16K capacity.
I hope you enjoy playing it as much as I've enjoyed writing it

Thanks,
John
(corresponding thread on Spectrum Computing: https://spectrumcomputing.co.uk/forums/ ... hp?t=13275)