They were building it anew.
There hadn't been much left after the devastation, and the city they'd lived in had been a stinking, rotten, fetid ruin. Better to leave it to the dead and dying, better to leave the diseases to work their course. Those of them who could walk, who could carry a pack, who wanted to live, had banded together and headed for the hills.
They had among them a surprising number of skills for "city people," and just as surprising, to them, were their gaps in knowledge, vast holes that the city, that civilization had filled in. but they had what their ancestors had had, in spades: a strong desire to keep living, and a willingness to innovate.
They didn't all have a willingness to work hard, but those who didn't either fell by the wayside, or found ways to work "smart," to reinvent old technology quickly, and to steal or jury-rig what they couldn't just make.
They were on an old tor, where thousands of years ago, castles had risen from the ground. The castles were gone, victims of age, victims of the same devastation that had ruined the city, but the things that had made the tor a good place to build a castle were still there: fresh water, a view for miles in every direction, and stone. Stone and stone and stone. They grew tired of looking at stone, of carrying stone, of cutting stone. Their hands were covered in so much dust that they might as well be stone. They dreamed of stone.
But they dreamed in safety, behind sturdy walls that grew sturdier and safer every day; they dreamed with full bellies, their food supplies growing from hunting and gardening and plain old scrounging; they dreamed in a growing community, in a world they were building for themselves:
Their dreams were of stacking stone; but their hopes were of stacking knowledge.