Left side of the case again.
You can see pretty clearly the intended use of the window :)
As well as how the internally mounted 120mm intake fan draws cool air in from outside the case.
There is a 1" by 5" vertical hole right through from one side to the other with mesh infront of the fan to avoid stray fingers from getting chopped, and the PITA* fan from getting broken and having to be replaced.
* Pain In The Ass : The fan is mounted in a shroud that is held in place by 4 TINY screws in a VERY inconvenient location if you are to ever replace/change the fan.
I will at some time mod this shroud with a hinge and a single normal case screw so I can easily replace/clean the fan.