This work would not be possible without the efforts of:

Philip M. Anderson — The Complete Through the Trap Door RAM Disassembly (2015–2021)

Used as a guide to the layout and the Don Priestley engine family (it covers the 1987 sequel Through the Trap Door, not this game). Made with SkoolKit.
https://wolfe-lyon.github.io/disassemblies/through_the_trapdoor/