Due to hasard, I found the Github of Jason Brooks (Argonaut) where you will be able to find the source code of his Amstrad CPC programs (1985 to early 1990). You can read there that he was mostly programming in assembler, ADAM (Assembler, Disassembler and Monitor) was his tool of choice as it was fully relocatable, could be loaded into a ram bank on a 6128 and was very capable when hacking and reverse engineering code and protection systems in its day