What differences in programming Spectrum and CPC

[PeterJ]

You might want to read these tutorials:

https://www.chibiakumas.com/z80/

They were written by a developer who had a great idea of writing a modern shoot up for multiple platforms. He used to be here, but sadly took offence to what was in all sense and purpose constructive feedback regarding his game, and therefore left.

[spectron]

I only did one conversion from Speccy to Amstrad (Gunboat by Accolade).

That was fairly easy because it was solid(ish) 3D so most of the drawing was just horizontal lines.