I think I've stumbled on to a bug that stops me from finishing:

I've found all the level keys (I see them all in my inventory), but for some reason the final stone pillar in the Main Hub refuses to go down and I get the "Level 6 Key Required" notice on the screen. I've tested it a couple of times and it's always the final pillar and is not depended on an exact one.

I've attached a screenshot of it.