First of all, thank you very much @Springheel & Sotha for your video tutorials. These are extremely helpful (sometimes even funny), and for the very first time I was able to create a small area mapping (almost) to my expectations. I understand that visportals are essential to the entire process and before I continue adding more details, I would like to solve this problem first. Regarding visportals and window models surrounded by brushes, the WIKI tutorial (written by Sotha?) recommends to "put in a worldspawn brush with nodraw texture on". So I did, and I carefully located these "No-Draw" brushes between the window prefabs in question to make sure that there are no gaps left (I also double checked with grid snap and carried out visual control via DR). Result: "r_showtris 2" shows that the entire outside area of the building is still being rendered, fps down to ca. 10 - 15 fps. If I replace the window prefabs (brushes, windows and no draw brush) and replace them with solid wall brushes, it works perfectly and the fps rate is sky-rocketing up to 50 - 60 fps. Thus I think that the "no draw" brush does not seal my wall works at all. What is my mistake? Please find below a picture; the red frame in the detail depicted on the right side shows the exact position of the no draw brush. Many thanks for your help in advance.