Texturing & Skybox
To make a quick ceiling select the arrow tool and click on the floor in the camera view to select it.
If you can't quite see the floor put the mouse pointer over the 3D Window and use your keyboard arrow keys to redirect the camera
Once the floor is selected move the mouse pointer to the plan view and Ctrl+C then Ctrl+V to copy/paste it.
Move the ceiling into position remembering to check the side view to get it placed properly
We now have a sealed room with no leaks, but it looks strange with the same texture all over
so select a wall (click in 3D view)
Tip: You can save time here by selecting all 4 walls by holding down the Ctrl key and clicking on them one at a time
Now select the Texture Tool (green, blue, orange cube)
When the texture window pops up click on "Browse"
Pick a wall texture you like the look of and double-click it
Rather than a boring ceiling let's make it appear like it's outside with a skybox texture for the ceiling
Select the roof and open the texture tool again and hit "Browse"
Type Sky in the Keywords box and it'll narrow the textures down to Skyscraper (annoying isn't it)
Scroll down to the SKYBOX textures and double-click one
Now all we need is a light source so we can see inside the map
Select the Entity Tool (light-bulb shaped thing) and click inside the room in the plan view and hit enter
You will probably have to move it in the side views to get it in the room properly.
Now right-click the new entity and select properties and change it to a "light" entity if it isn't already one