Thanks for the rec! I unfortunately don't have a Speccy, but I did find an emulated version of the BBC Micro port of Twin Kingdom Valley and can definitely see it drawing in the various pieces of the environment.

https://bbcmicro.co.uk/game.php?id=89

I think the approach of using simple procedural vector graphics was relatively common back in the days when you had to optimize for either disk space or speed. Sierra's creators talked about how they opted for procedural graphics to fit their earliest games on the old 80 KB floppies.