I usually go through great pains, when designing dungeons, to make corridors that are at least 5' wide, and often include many that are 10' wide. The logic, of course, is purely gamist and has nothing to do with the considerations of the structure of the buildings. I want fighters to be able to fight.
All that is about to change. Moving through medieval structures in Florence has given me a better appreciation for the size of rooms and stairwells. Indeed, most medieval buildings don't have corridors at all, but rather rooms opening onto other rooms. However, I preserve them for dungeons on the grounds that most of these places are built in a substantially different style to the "modern" world of the setting. That being said, chokepoints conveniently occur at doors and stairwells in medieval buildings.
Now, stairs... I have never imagined a stairwell or a corridor of less than five feet would exist in a medieval building. I had never been inside the winding, twisting, crazy path of the Duomo stairs. They are easily 3' at their widest and continue upwards for 462 steps. The walk is grueling, and the space is suitable only for grappling and stabbing. Even a pike or spear would have a hard time being used in such confined quarters, as the stairs turn and turn and turn on themselves, leaving no room for the butt-end of the weapon to trail or be set.
So you'd better believe that narrow design is having a comeback. Roofs that are too low for standing up (like the excavated crypts and church below the Florence Baptistery, which is a great model for a dungeon), floors that are sharply canted, and all other manner of architectural nightmare should soon find its way into the 10th Age. Farewell to the spacious, simple, rectilinear dungeon design. Hello, ancient and medieval architecture.