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The Castle is replaced by the following unique building:
Strategy
Castles increase your city's defenses against pre-gunpowder units, increase espionage output and generate a small amount of culture. Build them in cities near enemy civilizations.
Civilopedia Entry
Castles served many functions in medieval Europe. Not only were they formidable defensive positions, but they served as administrative and judicial centers and as the seat of a lord's power. Since food and supplies could be stockpiled at castles, they exerted strong influence over very large surrounding territories. Secular lords were not the only ones to build castles; religious officials often did so as well, to provide themselves with stronger power bases. Early castles were usually little more than wooden stockades constructed on dirt mounds and perhaps surrounded by a shallow trench; by the time of the reign of Edward the Confessor in 13th century England, castle-building had become a much more elaborate affair, with hundreds of laborers and specialists constructing huge stone fortresses with intricate defenses in carefully selected geographic locations. After gunpowder brought cannons into widespread use, castles failed to provide the same protection from attack, and later rulers built palaces more along aesthetic than defensive lines.