Most creatures must be played by a Base of their Domain.
Dwarven Hall is an Example of a Mountain Folk Base.
Note that not all buildings that are bases give you domain and not all buildings that give you domain are bases. You see here that the Exhausted Mine is a Mountain Folk base but doesn't give you Mountain Folk domain. Whereas, this Hadarck's Thrown gives you Mountain Folk domain but isn't a base.
Unaligned Creatures can be played by any of your bases.
Unless a creature says otherwise it must be played next to a base. If the creature must be played elsewhere it will specify on the card. It will say "Spawns some type of location."
For example this Alerwyn Barque Spawns in oceans. So you can play it in any ocean that you can see regardless of if you have a Sylvan base or not. The only other requirement is that you can't spawn it in an enemy controled ocean. If something doesn't spawn by a base it can't spawn in enemy controled territory.