When I 'wrote the script' of the level, I had a very firm idea of what that was and why it was there, but it wasn't at all obvious when the level was finished. For the release, I added a secret room to that area to explain its presence. I guess you didn't find it
Ah well. It was fun making that part, 'cus it's very different from the rest of the level and I was so bored of tunnels and caves by that point
Still, glad you enjoyed it so much, and tracked down the bonus ending!
My next level I have planned will have probably four bonus endings. It's plot is a lot more complicated than 'Eurydice', and since I'm no good at using flags it's gonna take a lot of copying, pasting and reprogramming warps.