Growth
When you aquire acres of land you will need to put buildings on them.Each type of building (except homes)will employ your peasants to make the buildings have a positive effect on you province and the employed peasants will generate income in the form of gold coins (gc's) too, great isnt it!
Your buildings will also allow more peasants to occupy your lands meaning you can have more armies.
To make sure you get your buildings to work effectively for you, you need to have a strategy (build strat) as just building anything or everything will not aid you in the long term, check the link to Utopia strategy guides, find your race and the best rated build strat and start there, as you get more familiar with the buildings and your kingdom mates you will learn more and may decide to tweak the build strat a little
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