How was New Orleans Founded?

If New Orleans borders Lake Pontchartrain as well as the great Mississippi river, while being below sea level, how in the hell was it founded? How could one possibly build a city that by all means should be below water? It boggles the mind. The levies couldn't be there in the 1700's, could they? How does a city, that is below sea level, get built in the first place? Especially back in 1718?>