Some city improvements must be maintained after they are built. This maintenance cost is paid from your treasury once every turn. These buildings require one gold per turn. Aqueduct, Bank, Barracks, Civil Defense, Courthouse, Granary, Harbor, Library, Marketplace, and Temple. These require two gold per turn Airport, Cathedral, Colosseum, Commercial Dock, Hospital, Mass Transit System, Police Station, Recycling Center, Research Lab, SAM Battery, and University. These require three gold per turn Factory, Manufacturing Plant, Offshore Platform, Stock Exchange, and all Power plants.