building on the "whole universe is one town/island/small area."
The whole universe is very small, maybe a diameter of a kilometer or two, but extends infinitely down and up. Since the horizon loops and people can see themselves infinitely, even if smaller and smaller, it disturbs them, so they figure out where the universe "ends" and they build walls without windows around the perimeter. As their society has progressed and grown, they have had to extend up to accommodate the growing population. They mine the necessary metals and other building materials from the ground.
This leads to three things, first, they become incredible architects, capable of building sturdily soaring structures. Second, there is no war, for two reasons: first, the close proximity everyone lives to each other; second, though their world (they think of the tower as such) is almost impenetrable, the bombs and explosives they use to mine their materials become advanced faster than their building materials, so they always worry about destroying a level of the tower and killing everyone in it. The third idea is that the open air only extends so far, say 100 or 150 km up until suddenly gravity reverses itself and up becomes down. Anyone crossing this threshold would immediately fall to the ground on the other side, which is the same 100 or so km down.