How to make a window like the one on the first blu spawn on swiftwater?

[Fillie]

Ok so I'm making my first ever map (for TF2 at least) and I had an idea to make a window in blu spawn so they can see what red is doing during the setup phase like on Swiftwater. Only thing is I don't know how it works, so if there's a solution to this please say so.

 

[Seacat08]

Easiest way to do it is to have a transparent glass texture on the side in Blu spawn and to have an opaque texture on the side red can see.