Firstly, thanks for the incredibly detailed breakdown. I really apprecaite it. That gx gy graphical coordinate trick is great! I was so focused on calculating the absolute pixel math on the fly every frame that I missed the optimisation, just pre calculate the starting pixel and increment it by 32 or 16 when moving! Eliminating those multiplication cycles to the point where you don't even need FRAME anymore is exactly the kind of magic I love about 8-bit BASIC coding.
I originally used INKEY$ just to easily catch both QWERTY and AZERTY keyboards, but you are right that INKEY() or JOY() is the proper way to shave off those cycles.
Thanks for the heads-up on the cavern width variable (w>2). I'm learning a lot here. I'm by no means the finished article. Much appreciated. I don't think I can submit an update now, with your help, fair play mentioning 1.1 only that prompted me to do a 1.0. When the competition is over I'll put a super optomised version on my account and will credit you with suggestions made.