OK, starting with the basics. Here are the various city improvements, their maintenance costs, and their functions.
Improvement(Maintenance Cost): Function
Airport(3): Allows one Airlift per turn to and from another Airport: City produces Vet Air Units: 1 turn repair for damaged units: Increases trade route potency
Aqueduct(2): Allows city to increase beyond size 8: Can sell it when city is maxed out in size
Bank(3): Tax/Luxury Output Increased 50% (*** with MarketPlace): Don't build if city has less than 6 coin output
Barracks(1): City produces Vet Ground Units: 1 turn repair for damaged units: These are made obsolete and must be rebuilt
Capitalization(0): Converts Production into Trade
Cathedral(3): Makes 4 unhappy citizens content (3 after Communism)
City Walls(0): 3x Defense for Ground Attacks
Coastal Fortress(1): 2x Defense vs. Shore Bombardments
Colosseum(4): Makes 3 unhappy citizens content (4 after Electronics)
Courthouse(1): Corruption decreased 50%. More bribery resistance. Under democracy, makes 1 content citizen happy
Factory(4): Resource Production Increased 50%
Granary(1): Only 50% decrease in food supply after a population increase
Harbor(1): All ocean squares produce 1 more food
Hydro Plant(4): Factory Output Increased 50%, cleaner than power plant, safer than nuclear plant
Library(1): Science Output Increased 50%
Manufacturing Plant(6): Factory Output Increased 50% (*** with Factory)
MarketPlace(1): Tax/Luxury Output Increased 50% (*** with Bank): Don't build if city has less than 2 coin output
Mass Transit(4): Eliminates population based pollution
Nuclear Plant(2): Factory Output Increased 50%, cleaner than power plant, same as hydro plant, but there is meltdown risk
Offshore Platform(3): All ocean squares produce 1 shield
Palace(0): Eliminate corruption and waste, reduces it in neighboring cities: Immune to bribery
Police Station(2): Decreases unhappiness from troops away from city by 1
Port Facility(3): City produces Vet Naval Units: 1 turn repair for damaged units
Power Plant(4): Factory Output Increased 50%: Wait for Hydro or Nuclear (if you can) to avoid pollution problems
Recycling Center(2): Decreases pollution caused by factories
Research Lab(3): Science Output Increased 50%
SAM Missle Battery(2): 2x defense vs. Air units and non-nuke missle units
SDI Defense(4): Nuke protection for everything 3 spaces from city
Sewer System(2): Allows city to increase beyond 12: Can sell when city is maxed out in size
Solar Plant(4): Factory Output Increased 50%, cleanest form of power
Stock Exchange(4): Tax/Luxury Output Increased 50%: Don't build if city has less than 8 coin output
Superhighways(5): All roads produce 50% more trade
Supermarket(3): Farmland squares produce 50% more food: Don't build unless you have farmland (double irrigated squares)
Temple(1): 2 unhappy citizens made content
University(3): Science Output Increased 50%
Improvement(Maintenance Cost): Function
Airport(3): Allows one Airlift per turn to and from another Airport: City produces Vet Air Units: 1 turn repair for damaged units: Increases trade route potency
Aqueduct(2): Allows city to increase beyond size 8: Can sell it when city is maxed out in size
Bank(3): Tax/Luxury Output Increased 50% (*** with MarketPlace): Don't build if city has less than 6 coin output
Barracks(1): City produces Vet Ground Units: 1 turn repair for damaged units: These are made obsolete and must be rebuilt
Capitalization(0): Converts Production into Trade
Cathedral(3): Makes 4 unhappy citizens content (3 after Communism)
City Walls(0): 3x Defense for Ground Attacks
Coastal Fortress(1): 2x Defense vs. Shore Bombardments
Colosseum(4): Makes 3 unhappy citizens content (4 after Electronics)
Courthouse(1): Corruption decreased 50%. More bribery resistance. Under democracy, makes 1 content citizen happy
Factory(4): Resource Production Increased 50%
Granary(1): Only 50% decrease in food supply after a population increase
Harbor(1): All ocean squares produce 1 more food
Hydro Plant(4): Factory Output Increased 50%, cleaner than power plant, safer than nuclear plant
Library(1): Science Output Increased 50%
Manufacturing Plant(6): Factory Output Increased 50% (*** with Factory)
MarketPlace(1): Tax/Luxury Output Increased 50% (*** with Bank): Don't build if city has less than 2 coin output
Mass Transit(4): Eliminates population based pollution
Nuclear Plant(2): Factory Output Increased 50%, cleaner than power plant, same as hydro plant, but there is meltdown risk
Offshore Platform(3): All ocean squares produce 1 shield
Palace(0): Eliminate corruption and waste, reduces it in neighboring cities: Immune to bribery
Police Station(2): Decreases unhappiness from troops away from city by 1
Port Facility(3): City produces Vet Naval Units: 1 turn repair for damaged units
Power Plant(4): Factory Output Increased 50%: Wait for Hydro or Nuclear (if you can) to avoid pollution problems
Recycling Center(2): Decreases pollution caused by factories
Research Lab(3): Science Output Increased 50%
SAM Missle Battery(2): 2x defense vs. Air units and non-nuke missle units
SDI Defense(4): Nuke protection for everything 3 spaces from city
Sewer System(2): Allows city to increase beyond 12: Can sell when city is maxed out in size
Solar Plant(4): Factory Output Increased 50%, cleanest form of power
Stock Exchange(4): Tax/Luxury Output Increased 50%: Don't build if city has less than 8 coin output
Superhighways(5): All roads produce 50% more trade
Supermarket(3): Farmland squares produce 50% more food: Don't build unless you have farmland (double irrigated squares)
Temple(1): 2 unhappy citizens made content
University(3): Science Output Increased 50%
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