Nice game!
Learning my way around the tunnels and slowly building up the knowledge of how to progress safely was fun. I liked how you can only see the ghosts when they were close and otherwise you have to look for smaller signs like footsteps.
Was able to ultimately find all of the secrets and reliably get to the boss chamber unharmed, but I just couldn't beat him. Maybe there were signals that I was missing, but I couldn't get a handle on a strategy that would let me hit him reliably. I think it's possible that having to do the tunnel section first before every attempt was just enough to keep me from remembering how I had approached the fight last time, so I felt I was probably just using the same failed tactics over and over without really learning how to succeed. I think a checkpoint before the boss fight is all I needed!
Really loved the sound and music design too. I thought you used them to set the tone magnificently, and the musical shift immediately informed me when exploration was over and it was time to fight without any other indicator necessary. Very effective!