I was involved in the design and play-test of each of these levels. Please know these were created from *scratch* and we spent our time making sure the levels were as playable as possible. We were not terribly focused on texturing, other than to give players positional awareness in different parts of each arena.
Map 6 -- Offices of Death -- Was intentionally designed to be as accurate a representation of the office we worked in as possible. Admittedly, playability was taken as a second priority. We did manage to create working "elevators" and "stairs" to give the illusion you were going up and down floors. I was pretty happy with how we did.
Considering it was 1996, and no way to simply "Google" different ideas. Your best bet at the time was to jump onto the Usenet and hope you could find a decent board with active members. Other than that, it was pretty much all self-taught.
I'm glad you enjoyed the other levels. Yes, they are old-school, big arena, style. We spent a LOT of time trying to make sure they were as balanced as possible, with escape routes, and required risks for rewards.