I believe how it works is that there are building plots that you can choose to build different buildings on. There will be different places where you can put walls so castles in the same area don't mirror each other. I would imagine that on building plots that aren't for walls, you will be able to put like either a house or barracks or warehouse or something like that, for humans at least.
For elves, you can put stuff on trees and the walls will be somewhat similar to humans but would have to connect to trees at some point or something. You will probably be able to choose between different plots where some stuff will go.
For orcs, build whatever wherever.
I could be totally wrong, though, please correct me if so.
Picture made by Aametherar the orc (Elves are the best!)