Inventing secrets for our settlement can be fun but is optional. Waiting until we have a need for a secret is recommended because a secret is, by definition, unknown, and we’ll have no reason to mention it unless characters uncover it. Secrets can be incorporated into city layout, such as there being a building off limits to most people, for a reason that’s a lie or which isn’t admitted to at all. Maybe a building’s location seems inappropriate, and the explanation lies underground, such as an entrance to secret catacombs, or a supernatural phenomenon, whether sinister or helpful. If we’ve created gods, world figures, and undead, the events of their existence can result in such places.
Secrets can be more mundane, such as benevolent or nefarious groups operating in the shadows; they might have influence over settlement leaders. Hidden doors or places are less exciting unless there’s something truly interesting there, such as a portal. We can have landmarks like statues that have an unknown feature that is activated under the right circumstances. Maybe a statue is a being that can be reanimated, either as a stone golem, a living person, or undead. Religion is good for creating places with unknown (or largely unknown) features. See chapter 11, “Creating Places of Interest,” for additional ideas.