Production buildings can be constructed in the small building slots, with the exception of Stardust Mills that occupy large building slots.
You can construct up to 10 small-slot production buildings. Keep in mind that there are only 19 small slots available in your city, so you cannot construct them all!
There are four types of production buildings: Lumber Mills, Mints, Quarries and Stardust Mills. Houses provide living quarters for your units, allowing you to maintain larger armies.
Lumber Mill
Lumber Mills produce Wood and defend your Wood supply, should your city defenses fall. By upgrading your Lumber Mills, you increase the amount of Wood produced, as well as Wood protected if an enemy plunders your city.
Requirements: Citadel
Required for: Workshop
Mint
Mints produce Gold and defend your Gold supply, should your city defenses fall. By upgrading your Mints, you increase the amount of Gold produced, as well as Gold protected if an enemy plunders your city.
Requirements: Citadel
Required for: Forge
Quarry
Quarries produce Stone and defend your Stone supply, should your city defenses fall. By upgrading your Quarries, you increase the amount of Stone produced, as well as Stone protected if an enemy plunders your city.
Requirements: Citadel
Required for: Warehouse
House
Houses provide army capacity to your city. Each unit has its own weight, and extra Houses allow you to recruit and maintain more units at a time. Once your city capacity is reached, no additional units can be recruited. Keep in mind that your city capacity takes into account all units present in your city, on your wall, in your recruitment queues in the Barracks, as well as in any camps outside your city. At the same time, units being healed in your Hospital or reinforced through your Embassy do not add up to your city capacity until they become available for use (until healing queues / embassy orders are completed). You can go over your city capacity by using Spheres of Summoning to get extra units or by healing units in the Hospital once your city is at maximum capacity. This will put you over your city capacity, and while no additional units can be recruited, you will suffer no penalties.
Upgrading each House increases the amount of capacity points it provides. It's recommended to build 10 houses first and keep them as upgraded as you can, as your army is the main "resource" in the game. Additional resources can be mined from the Outposts on the global map.
Requirements: Citadel
Required for: Barracks (and subsequently Wall, Hospital, Watchtower and Citadel)
Stardust Mill
Stardust Mills produce Stardust and defend your Stardust supply, should your city defenses fall. By upgrading your Stardust Mills, you increase the amount of Stardust produced, as well as Stardust protected if an enemy plunders your city.
Requirements: Citadel
Required for: University