Back in May 2015, I spent an evening watching retro game documentaries and was inspired to develop a game for the ZX Spectrum... a computer released back in 1982. After all, with everyone talking about making retro games (but actually making modern games with retro visuals) I decided to make a real retro game. In machine code. This talk covers the methods by which a modern computer is able to help developers write, debug, and test new software that runs on old systems. It uses the game "ZX Adventure" as a real-world example. Old machines had so little power you were forced into writing highly-optimised assembler if you wanted anything to fit into that tiny memory, or execute reasonably fast on those slow processors. Luckily, modern machines make the process of writing software so easy you can spend all your development time on the fun parts - and it's this part we'll focus upon. Starting with the game design, we begin by creating "Hello world" on the obsolete machine, understand the truly minimal requirements, and building up the game piece by piece. We also touch on the issues of emulation, licenses, and publishing, given that the original Spectrum game has now been ported to the ZX81 for a commercial (and yet still open) release.