with that portal thing is meant, that the bsp-tree is build-> visibility calculation.
So under that circumstances, the level have to be not too complex and should have no major details(stones ,fences and that stuff). In best case only the bottom, housewalls and the roof.
All the details you want to add to your level you should do with models( you can also give an easy ffp-shader, so that they look exactly as the level).