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Zyngas idea of "El Dorado" appears to have come from their new game adventure world for which the last location you visit is called "El Dorado" and both locations here on mafia wars and adventure world have the same Aztec style locations.
Reply - "El Dorado" was the name of a Native American chief of whose story evolved into a mythological city of gold that was searched for by the Spanish conquistadors in South America, with much of the search taking place in the Amazon. The locations used in the depiction in Mafia Wars are roughly appropriate for a city of that region and time frame.