It depends on the part of the level. I break the parts down its easier to manage. The floors and wall like a dungeon 3dws would be fine. However when it comes to adding things like buildings and other objects importing is better way of doing things. This is 3DWS is a world building program and not really modelling one. Personally my working method below.
1. Make a list of rooms
2. Make a list of what’s in each room.
3. Draw items for each room
4. Model items for each room
5. Draw a basic room
6. Make room
7. Put everything in
8. Walk room
9. Move items until happy
I'll add something later on.