In general, fans should almost always try to keep the cities that they capture in Civilization 6, as they are worth, at the very least, the Production associated with building a Settler.
After a player has successfully captured a city in Civilization 6 they will be confronted with a big question: Should they keep the city or should they raze it? As with many of the decisions that arise during a game of Civ 6, the best way to proceed with this one may not always be immediately obvious, and this guide looks to detail what players should be thinking about during their conquests.