Archon ---> Quake. You heard right, elements of the 80's micro game archon extrapolated into the Quake universe. Get ready for a wild ride. (free stuff | no warranty | see disclaimer.txt | blah blah blah.)
Normally quake 1 does not do "transparent" textures. Even with advanced engines.
Windows in quake 1 are usually "clip" texture. Players cant go through it but weapon fire can. You cant see it like a glass pane either...
But I discovered a trick with darkplaces. You make a regular texture, stuff it in a texture .wad and build the map. Then you make an override .tga texture that has transparency. Voila - windows. The texture will have some level of transparency (set by the alpha level of the texture in the .tga) from barely see through to near transparent glass.
There is one caveat. Vis sets visibility for entities - it doesnt "know" players should see "through" the glass. So you can see the walls on the other side of the glass, but not entities. (The glass edges can also look weird where they touch other brushes too. (You can see through the edge to exterior parts of the map.)
If you leave a small crack around the glass (like here) you eliminate the see through map, provide a small possibility of seeing ents on the other side (not well in this case - you usually dont see them) and have that cool shadow effect at the glass edges.
Trick for a future update - "shootable" windows.