TAXES turn your cities' TRADE into gold for the treasury.
On the tax rates screen you split trade between taxes, LUXURIES, and SCIENCE.
Gold pays upkeep for units and buildings, and can rush-buy.
Run dry and your treasury drains; empty it and buildings are sold off to cover the shortfall.
Marketplaces, Banks, and Stock Exchanges raise a city's take.
Your government caps the tax rate.