ADAM SMITH'S TRADING HOUSE makes your empire's commerce self- sustaining. Trade revenues pay the maintenance of every city improvement that costs 1 gold per turn.
It also reroutes INFRASTRUCTURE
BONDS: cities producing bonds pool their shields each turn, and the pool is split evenly among all other cities.
Adam Smith argued in The Wealth of Nations that individual ambition, operating through free markets, generates collective prosperity. Cities that traded freely produced enough surplus to fund their own infrastructure without royal edict.