Houses

From The Five Pillars

The biggest part of your kingdom will be transformed into Houses.
Without houses you can not have population and without population you cannot have a gold income.
Your population will breed until all houses are occupied, after that it will just go up and down a bit.

Houses have an upkeep, every House you own will cost you 70 gold each turn.

But what is a good number of Houses on your land?
In theory you should be able to have enough Houses to keep your gold reserves under control.
30-40% Houses on your land is a normal amount for a mage.

Remember, not your Houses give you gold, your population does! So make sure all your houses stay occupied.