A Plaza is a building that increases the rate at which you gain culture from luxuries along with Capitol. Plaza can be accessed by researching Plaza during the Classical Age. In the Renaissance Age, the plaza is replaced by a Market. To research the Plaza you need Governance, Library 4 and 200 Gold.
|6||30||?||Town Hall 1||-||Feudal|
Jagex, released some pre-release screenshots in their facebook page, along those images there was an Ancient Plaza, but this is not in-game.