A Great Wonder can become a Tourist attraction once it has "stood the test of time" for over 100 turns. A tourist attraction then begins to attract commerce to the city in which it is constructed, which steadily increases as time passes and its fame spreads. An attraction that has been built for over 100 turns but less than 201 turns produces two gold for its owning city; one that's been around for over 200 turns but less than 301 produces four gold, etc.