I did some more research into the concept art’s depiction. And I have found what has to be near definitive proof that we are seeing a scene from Sangheilios. Likely the Arbiter’s Keep.

Sangheilios is part of the Urs system. The Urs system is comprised of a white primary star of the same name. The other two stars orbit the primary one. Their names are Fied and Joori.

There are four planets within this system as well. One of which is Sangheilios. So there it is. The concept art is showing a scene from Sangheilios. The look of the buildings relative match those for the Vadam Keep.

Vadam Keep (the Arbiter’s keep) is built into the lower slopes of the Kolaar Mountain, with warrens running inside the mountain itself. If you’re not sure what warrens are, there are a couple of missions within Spartan Ops (latter half of the season) on a map by the name of Warrens. Check that out and you’ll have a much better understanding of what warrens are.

-Sal

Source Links
Urs
Fied
Joori
Vadam Keep