I was thinking like I-beams and H-beams, I think steel studs would be great too, but probably to much detail for something that will be covered up by walls and such.
The beams would be great for buildings as well as bridges and platforms, adding a realistic touch.
"Old people doing stuff...not me, I'm just old and think about doing stuff..."
Studs would be good for tall medieval gates and doors you can easy make I-beams and H-beams in your construction using blanks as showing below