I've seen this in movies or cartoons sometimes, that they are waling around inside the curtain wall, but hollowing out the curtain wall with a hallway seems like it would lover the structural integrity of the stone structure, that is supposed to withstand poundings from catapults and trebuchets. I noticed this in the excellent book Stephen Biesty's Cross-Sections Castle, where you can see it illustrated by the author, this is where I first relized the potential structural problem with this. You can see one example from this image below.
I've looked around on the internet but haven't been able to find any other image sources, like photographs of real castles, that confirms that corridors in the curtain wall was used. Does anyone have any insight into this? Perhaps someone who has actually been to some real castles, unlike me(!)