Note, that I never mapped much, but my standard approach was generally to cut a few large rooms out in the general shape of the map(because despite the clever opening if you got too ambitious too quick unreal absolutely would leak like a sieve) then use a mix of add and subtract brushes to make the fine geometry, which was exactly the same as quake maps, large simple rooms to avoid leaks(this time needing to be assembled) filled with fine geometry via adds/subtracts.
This is sort of the early id, Epic engines in a nutshell, id better technically, Epic better tooling and ergonomics.
And a footnote on brushes, because I did not understand this until long after I stopped mapping, A "brush" is a 3d primitive unique from the point,face one commonly used in 3d modeling software. A brush is defined as the convex shape made from intersecting planes. So where for a point,face model you define where the points are and which points form a face, for a brush you define where the face is(point, normal) and what faces are in the brush, the shape comes out of how they intersect. I assume this was because for large simple geometry that you are going to be doing csg operations on the plane model is more attractive than the point model.