National wonders are much like regular nation improvements in that they allow you to further modify your nation based on your own strategies. The difference between national wonders and nation improvements is that national wonders do not require a specific number of citizens to develop but instead have an enormous cost associated with them. Another difference to national wonders is that you can only purchase one of each type of national wonder and you may only develop one national wonder once every 30 days. National wonders cost $5,000 a day to maintain.