Just like Collection Points give up Trade Items to Civilizations that cover them, enemy Cities also spit up Gold for the Empires that encompass them, but you cannot Plunder Cities that are inside your territory.
In the event that your Priests have Worshipped and Taught so effectively that your Territory covers someone else's City, you are given the chance to play the evil Biblical tax collector and Tax the City once per day. All you have to do is click the City, select Tax, and Dispatch a Hero to collect an amount of Gold that is consonant with the quality and resources of the City in question. And that's it! Unlike Plunder, your Hero has no chance of being attacked during his or her journey of repossession, so there's no need to Scout a City before you Tax it. But do watch out---abuse the privilege, and you may find that your previously weak neighbors light some heretofore unseen fire under their pantseats, Dispatch a few Defaming and Teaching Priests, and and tear themselves from your hegemony to exact sweet, sweet revenge.