Originally I could only fit one level + boss into 128k, so the main concern was always memory. Luckily there is a type of compression called ZX0 which helps alot. I had originally cut the crushers and lifts in the warehouses and junk yard, as well as spikes and robots, but I somehow found the memory. The only thing for speed was reducing the number of enemies at one time so it didn't go too slow
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so u actually managed to bring back some of the content u originally had to cut! ZX0 compression sounds like it made a huge difference btw, i've really enjoyed chatting with u about your games haha would u be comfy continuing our little game dev convo another platform? no pressure at all if you'd rather stay here!