oh OK yeah, you would be able to tell that the building's surfaces were not parallel. if you had a hallway long enough, and you looked down it, you wouldn't be able to see all the way down the hallway because of the curvature. everywhere in the hallway though, it would locally be 'flat' - a marble on the floor wouldn't roll, assuming it was built to be level everywhere along the hallway. on the other hand if you only measured "level" in one place, and built the hallway perfectly flat and parallel, then the floor would not be level locally except where you measured it, and at some distance the angle the hallway floor made with local vertical would be big enough that a marble on the floor would roll towards the spot you measured "level" at.
