I have no idea what to do here and how to pass the columns. If it's not obvious enough for me I guess that most players won't have the slightest clue what to do. It needs a hint.

As far as I'm aware, you need to kill all the enemies in the room to progress, including those colored cacodemons Gez had made.

To make this a bit more obvious, there could be two tall monoliths at each side of the doorway, inscribed with glowing runes. Every time a Cacodemon or a group of Imps is killed it could lower down several units? When the monoliths reach the floor that passageway lowers?

That's a good idea. BTW.... where are these cacos or imps? I killed everything in the room and nothing happened...

When you enter the room imps spawn in the cages. When you kill all the imps green cacos appear. Kill them and the passage is unblocked. The Shadows in the side ledges are not part of it, but are very, very annoying.

It's very hard to hit the imps and I don't like the fact that it's unclear that you need to kill the imps to proceed considering that they ARE INDEED hard to hit.

Yeah, originally the cacos were spawned only to repopulate the cages once the imps had been killed. I'm not convinced blocking the way until the cacos are killed as well was that good of an idea.

I like the monolith idea, but if this is to be done, the cages should really be lowered so the player can actually reach the things.

I wonder what it'd be like to make them 3D floor cages that adjust their height based on the player's....

Seemed a shame to have this arena go to waste, so I made it a forced fight. You'll be glad to know I've made it so the cacodemons get forced out of the liquid by a ThingThrustZ now, so although you don't see them spawn, you do make sure to fight them straight away. Swapping the stealth imps for something more visible would be my other suggestion to make the fact that there are things to kill more obvious.

STEALTH MONSTERS?!
Gez how could you