Building a simple cell room is not that trivial as it can seem at first sight.
To be sure that everything is correctly aligned you cannot rely on numbers or your ability to move the mouse correctly, you must use some special tricks.
Here a simple recipe:
1\ Rez a cube 10x10x.2 as floor (color it in blue). Blue means it is aligned on the z coordinate
2\ Rez a cube 4x10x.2 as wall (color it in red), y rotation must be 270 because it is aligned on the X coordinate, and it can be hollowed naturally, hollow it to 60%
3\ Rez or copy a cube 4x10x.2 as wall (color it in green), X and Z rotation to 270, so it is aligned on the y coordinate, hollow it to 60%
4\ set ruler mode to World (IMPORTANT)
5\ set grid interval to half the thickness: .1
Now position the blue floor and be sure it is aligned on integer coordinates.
Align on it the 2 red walls using the grid.
Align the 2 green walls using the grid.
Duplicate blue floor to have ceiling.
You should have produced a nice simple room with empty windows.
Link everything and name properly…
This room has to be properly completed.
==> missing a side with a door
==> missing actual windows
==> missing textures