This treehouse structure is currently the home of the Mastros' restaurant chain's Las Vegas location. Mastros has four other steak and seafood locations in Southern California and three in Arizona. The restaurant inside this particular treehouse is called Mastros Ocean Club Seafood Restaurant.
You can enter the treehouse by using the escalator seen at left, or by coming in from the mall's second floor walkway. If you need valet parking you can use Crystals' by looking for the signs in the front-entry driveway of City Center that read 'Crystals and Veer', which lead to the underground valet.
Photo shows Mastros 'mouth-like entryway' at right.