You only need to use the world brush and vector tool to build shapes like these.
1. Build the whole building with 4 brushes
http://imgur.com/FHIEPxp
2. Make the corners into 45 degree angles
http://imgur.com/sRUHBcu
3. Select the whole building
4. In your y/z graph select the top corners (using the vector tool) and move them towards the center of the building
http://imgur.com/a/VR0zn
5. Do step 4 for the x/z graph
Hope that helps!
Edit; I had a run around your map. There are a few places where it is very maze like. A general rule of thumb is don't have two hallways connect to each other, have them connect into a room. I drew two suggestions on how you could improve the layout. http://imgur.com/n9Mvtdu