In my newest playthrough, I can't access the master staircase to the Endless Paths from the keep grounds. The trap door only has an information pop-up about how it has been battered by the elements. I remember using it before, but I can't remember what might be needed for it. Is it a particular keep upgrade?

You have to access it from below before it becomes available. On the first floor of Cead Nua (where you find Maerwald), there is a giant Adra hand. You must climb up the ladder attached to this hand, and it will take you to the grounds entrance, which will then become available for regular use.

I completely forgot about that. Thanks.