I killed Maerwald and taken possession of stronghold; near the room of Maerwald and near the "big hand" I can "see" the enter for the ruins with a closed door/passage, but I cannot enter. How can I do it? And the other door closed with energy?

Thank you for the help.

 

That one is the master staircase, which you can access later coming up from below. The stairs to level 2 of the Endless Depths is on the southeast side of that area.

 

Speak to the statue-woman in the ground floor of the Keep to dispel the energy barrier blocking the way to level 2. 

What he said^

What you referred to is the master staircase that lets you skip back past all the levels so you don't have to go up and down through a ton of loading screens. The entrance to level1 is through that doorway that was forcefielded before you speak to the throne-spirit.

 

Also a small correction - the ruins are called "The Endless Paths of Od Nua". Caed Nua is the keep that sits above it.

 

-Stigma