Geometry Brushes are a good way to mockup levels. In ancient times they where used to build the whole level but now static meshes are used instead. The brushes are found in the bsp tab in the place mode panel.
There are four basic primitives possible, Box, Cone, Cylinder and Sphere. There are also three types of stairs, Linear, Curved and Spiral . Some settings on each for it's size and form and some also have a hollow option. It makes it possible to turn a box into a room with walls, floor and roof.
Additive and Substractive