Still in 3D mode, point at the top of the third sector you made (the one inside the first). Give it a texture, then scroll the mouse wheel up a couple of times to raise its floor height, creating a platform. You'll notice its new sides have red exclamations, meaning they're lacking textures - paint them now. Then, point at its top again, hold down Ctrl and scroll the mousewheel to increase the brightness. You can also change the ceiling height and brightness if you like.