Domains are owned and operated by a Guild and its members working together to improve options available to the guild. Your guild needs to be level 2 to have a domain. For an individual player to access the domain, each player must be above LV 20 and in a guild.
The button near your map with the flag is how to access your domain. Domains can be divided into Guild Domain, Guild Hinterland, and Private Domain. You can construct various kinds of buildings in the domains, or add up a certain amount of resources. Each day you log on, you accumulate time points which can be used for upgrading your domain how you choose.
Private Domains are controlled by individual players, whereas the Guild Domain and Hinterlands are run by the Guild Leader and Vice Leader. With these Domains, players can construct buildings, gather resources, and level them up to unlock special items and benefits that the guild members can use.
If you leave the guild, even though you will not be able to access the Domain interface, your Personal Domain’s advancement will not be affected. You’ll be able to access it again once you’ve joined a new guild. However, if the new guild is at a lower level than your Personal Domain, you will be restricted to construction of new buildings based on the highest level the new guild domain will allow.