I played it on my Spectrum and Amiga. It’s one of those games that has a particular feeling, it always struck me as lonely and a little weird. TIL it was written originally for the Beeb. Also I didn’t know it was released as “The Sentry” in the US.

There’s some download links for the C64 on this page at the very bottom, scroll down quickly to avoid gameplay spoilers! Looks like there are other remakes linked there too.