Useful model but can you add soem screenshot how it looks when a unit is behind it and how you can control transparency?

It's quite simple. You need to put a texture and associate it with an additive filter with maggos editor. The texture becomes transparent and gives the illusion of glass.

Finally, we're able to install windows! It's clear that you're able to see through what people needed in the model section.

I wonder if it is possible to create a mirror in Warcraft but I guess not since there is no reflections from models?

It's not possible with warcraft III engine. You can recreate a room in front of the mirror that would mimic the action. As long as you see the other scene from the mirror it can be achieved. I used this system in an old map of mine: Silent Hill : Arizona - Warcraft 3 Maps - Epic War.com
Otherwise you could take a screenshot from the opposite side of the mirror. You add a little blurry effect on it and turn it into a .blp file and apply the texture in front of the scene. The scene would work decently. However you would need to do many indivudual textures for each mirror you use.

Hmm that's a nice idea for a system. I haven't tested the map but it would be a cool system I think to mark a rect and copy the actions on the other side, However you will need a complete room for it, so I guess it takes lots of space on the map.

It depends the size of the room. But it is quite easily done. You can see an exemple there:

A simple but useful model.