Source Engine optimizes rendering with the use of visportals. If you can't see visportal A while standing inside visportal B, it won't render. Hint brushes cut visportals, so you can adjust them yourself. (don't make the entire brush tools_hint, create a tools_skip brush and then color specific faces hint)
For your map style, it is recommended to put hint faces that touch the building roofs (more specifically, the world brushes).
In cases where hint faces and areaportals do not work, you can use occluders. In some scenarios, prop fade can be somewhat beneficial.
For now, I can recommend
Tiftid's guide on optimization.
Valve Developer Wiki articles that may be useful:
Visibility Optimization
Hint Brush
Areaportal
Occluder
