Design habit.... I'm used to making every access path three-wide for pathing and FPS reasons; making it three-wide means we need floodgates and levers to manually operate them, not doors. Wouldn't be hard for the next ruler to do; there's a surplus of mechanisms already.
Oh, well you can still use doors easily enough. First you build a door normally against each wall, and then next to where the center door goes, construct a wall. Then you can put the middle door in. You can remove the constructed wall and the door will remain standing. This would be great, because then we don't have to be afraid of anything-that-can-run-through-stone-traps. We only have to worry about anything-that-can-run-through-stone-traps-and-also-break-doors.
I usually use raising bridges. If I absolutely need the space and/or resource conservation, I just set it as one tile wide and however tall. Instant door, with the added benefit of potentially being useful for crushing stuff.
I love those, my latest fortress has a big entrance hallway with a drawbridge in it that I can use to smash stuff.
Sadly I've yet to have any sieges, so I've only smashed a rock once to make sure it works.

The doors airlocking the whole trap are super convenient for keeping my dwarves out of the way, though.