According to ZX-Retro Gaming, „this is a fully playable Quake engine with moving enemies running at impressive speed on a standard 3,5 MHz ZX Spectrum 128k. The engine also has support for 7 and 14 MHz, where the 14 Mhz version also has a second floor.

Mit Videos. (indieretronews.com)
Das Projekt steht zum Download bereit.
Technisch sicherlich beeindruckend, aber gut spielbar ist was anderes :)