Unused Buildings in Rise of Nations
Solution 1:
The El Dorado building is meant for the new world campaign which is available in the extended edition and should be in the gold edition (since it should include the original plus extended edition). It is meant to give a metal in a map with few mountains. The battle that contains El Dorado is the invasion of the territory located in the north west of South America where Venezuela is located.
The Statue of Lenin serves no purpose but is present in some cold war maps as decoration near Russian cities. I've heard you can make custom scenarios with such buildings.