I've been playing Rise of Nations for about ten years, since I was little. Probably the best RTS I've ever played.
> Citrus repairs nuclear-powered aircraft carriers.
> Titanium intensifies the Russian Winter.
> Sheep make your civilization smarter.
> War Elephants instantly transform into tanks.
> Horses make tanks cheaper.
> Papyrus makes it cheaper to invent electricity.
> Rafts with bonfires instantly transform into nuclear attack submarines.
> Bears make it cheaper and easier to create the Department of Homeland Security and institute the draft.
> The Aztecs can field tank divisions.
> The Mongols support their horse archers with fighter jets.
> American Hoplites and Cataphracts.
> You can bribe trebuchets to transform into howitzers.
Conquer the World Campaigns:
> Play as Alexander the Great; beat up Indians and steal their elephants.
> Play as an European or Native American power; beat up Carribbean islanders and steal their rum.
> Play as Napoleon and stop Lewis & Clark's nefarious expedition.
> Play as Mikhail Gorbachev, eliminate Jimmy Carter's NATO and steal Ronald Reagan's nukes.
> Help the Americans and Chinese fight the Mongols and Ottomans in World War I for control of Mongol-held SIberia.