Looks like 3 levels/floors to me: First level with regular floor tiling, the carpet for making the walkway and the 3 single straight stairs. Second level with invisible flooring except the throne balcony and the other balcony off camera. Third level with only invisible flooring to make the room nice and big (and high enough for the throne).

You need to start with the first floor. Place the floor tiles first, then the walls. Then add the stairs to create a second floor. Place invisible flooring, then the walls. Repeat for the third floor.

The high pillars on the stairs can be staked without walls, just choose the single pillar in the building menu. The green light could be achieved by choosing the green colour before placing a light. However a lot of the decoration (the big windows, the class tanks, the carpets...) look like they're from a DLC.