Abandoned Hideout

I've spent the last three days studying the cave and I still haven't solved the mystery.
It should have been a very normal cave, dug by an underground river, and yet there are so many strange things.
Who built those almost senseless walls? Their function is not clear, the cave is solid and they were not even necessary.
And those holes, those openings in the ground, do not seem to be the lairs of some strange creature.
I certainly do not dare to enter them, I am a scholar of ancient ruins, not a fearless adventurer.