Based on my understanding, Caed Nua is supposed to occasionally come under attack by forces sent up by The Master Below. This has never happened to me.

 

Is it something that was cut?

Is it something random that is also semi-rare?

If I clear a floor will creatures from the floor below not be queued?

I had stuff appear in my main hall on some of the "attack" events - I presume that's where they're supposed to come from. 

 

Also it plays into the endings you can get (based on having cleared the dungeon or not). 

I've had bandits attack the main hall but nothing coming up from Od Nua