>>1643009
Usually I don't start building with anything at all, except maybe a map name that can roughly inform things. The other thing is I usually start off by picking what texture theme I'm going to go with--techbase, hellbase, psycho hell, Doom 2 hell--and go from there.
I try to keep in mind that architecture and rooms should be interesting and varied, so I usually build a large room to start with and come up with some windows into other areas or build a big interesting structure. Romero said that it was good to give the player something cool to look at in the first screen of a level (if the first screen isn't a doorway, anyway, like E1M3 or E1M4) and if you look at his levels, this is generally true. Build something with steps or some kind of raised platform or something, and make it a weird polygonal shape that isn't just a rectangle or octagon. Make it a weird shape that doesn't have the "right" amount of sides.
Then I just build out from there, making a series of small, cramped hallways that lead into bigger, interesting rooms. I also usually plan for big "fights" throughout the level, and then tailor the map's weapon and ammo loadout to that.
The best way I think of what to add and what to change is to just stare at the map or play through it, and think, "There should be something cool happening here." I usually tend to draw a room and go, "There's gonna be a door here and an exit to a hallway here," even before I know what's behind the door and where the hallway leads. This way you always kind of have an idea of what you should do next--"I have to build that room where the yellow key door leads."