Despite your hope, there is not even any inherent symbolism; gravity is simply a coincidence.
You may build any for which you can afford the resources.
This really seems like a silly question but we can't figure it out from the rules.
We're used to Le Havre where you can only construct the top building.
But in Caylus, when you are building, for example, a wood building, can you dig through the available buildings and choose the one you want to build? Or do you have to build the top one?