I just try to think of where the level ends.
In my WAD you go through only 4 levels each with a different theme in an attempt at variety..so I thought about how those levels link together and what would be the most likely path of progression in my imagination.
You start of in a base...ok, but where in the base? A room behind the main foyer that you never see...ok cool, so it is a staff only door you are in, so behind the door you can't get through I wanted people to imagine the foyer or first rooms you might expect to see in a futuristic base. So that gives people a sense of a bigger world by giving them a blocked off door.
Then I tried to imagine what could be on level 1 of the base....so I made a computer room, which leads to a crate room, which provides stock for the military supply shop you go into. Another bit that branches off from there are some corridors that lead to the end of the level, but it is blocked off as the security gates are down and the demons haven't opened it yet. Hence why I put them on the console there, they are trying to open up the base. It is a techy sort of corridor with some exposed pipes due to the demons ripping off the wall fixtures.
Another room that comes off the crate room is a smaller corridor with some staff bed rooms for overnight shifts. These are not military bed rooms, but they are more for the scientists, technicians, cleaning staff and that sort of stuff. So I wondered where you could go next...so I put a dark corridor here that leads to a bigger crate room, and I put some decorative shutter doors there. This room has no exit through a door, the shutter door is another illusion to a bigger world, to get out you have to go through the vents to get to the blocked off exit.
And I made all my exists as lifts, that take you to different levels of the complex, I thought lifts would work better to make it feel like you are leaving the previous level for good and being taken to somewhere new.
MY level 2 resembles level 1 in it's base theme, but this more high security....you come out into a wide corridor with some fake doors (level 1 has a few fake doors too) but amongst them, some doors work. You go though another crate room which supplies another military supply shop. And branching off here comes a super computer room and a narrow corridor with some high tech work rooms. You then can see the door you need to go through but you need a key which means you have to back track a bit. You find key, back track then go into another corridor similar to the first one. Fake doors again, but real ones hiding access to the key are here. These rooms are where the offices and administration rooms are, as well as corporate/government board rooms. Once key gotten you then gain access to the super computer room, get the final protected key to another dead end corridor with fake doors...but to get through you go under the walls to the general's hidden rec room where you gain access to the sewer system and the level ends.
Then in the sewer level it is pretty straight forward....you go through sewage. Every so often you will find run down military barracks because space up top got a bit cramped. Here is where a nuclear fallout shelter sits, a hight tech sewer management complex with a big machine room in it and some sciency rooms...this is the government experiment bit. further on is more sewer and at the end of the level is the old sewer management system, disused and run down you will sometimes come across old remnants of a sewer system from before UAC came to the base...it is in one of the old systems you gain access to the final level.
The dungeons...I thought that dungeons, not only for another theme but for a likely place to actually exists there. The notion of my levels is so to be escaping to the mountains and after the sewers, a lift that takes you up to the old dungeons was an idea. So the general idea of the location isa base in the mountains, (level 1 and 2) built on top of an old base. Further down the old base and UAC base connect and intertwine in the sewers (level 3) and as the journey goes, you get to the dungeons...no involvement with UAC at all...these old cells have existed not only before the UAC base, but even the old base that UAC built on....the old base connects to the dungeons out of the builders exploration of the area....they made a lift that access there....but haven't really used them and UAC has just ignored the dungeons all together.
In the dungeon level you go though cells, courtyards and eventually the final door to the mountains. My levels aimed for a path of progression. All levels make use of fake doors and routes to give an essence of a bigger world. Like my sewer level will have blocked off sewer tunnels and the dungeons will have caged off corridors.
A bit of a long winded post, but I'm just trying to explain my idea of level progression in the level and as a whole trying to make the base somewhat believable.