The inner arches are like those of the famous mosque called "Dome of the Rock" in Jerusalem (also called "The Noble Sanctuary", heehee)http://www.musulmonai.lt/cpg/displayimage.php?album=3&pos=8
This style of arch was also done a bit later in the mosque at Cordoba in southern Spain:http://www.islamicarchitecture.org/ia/i/cmosque-Qordoba6.gif
Yes, when I saw the interior of Denethor's hall, I jumped up with excitement at all of the elements there.
I would *love* to know of the research and influences that went into designing Minas Tirith. This is my favorite setting in the whole LotR film trilogy. It seems like they were reaching into some very evocative periods of art & architecture history (or at least, it seems to me like they were..)