When it comes to hidden Cities you got a few options when the technologies of the rest of the world is at medieval levels. But when modern stuff like planes and satellites come in you are kinda stuck with going underground.
A Few options:
Magic, the go to for pretty much every problem if you are willing to use it...a magical barrier can function in any way the story teller wants it to (As long as it is kept consistent)
Superstition, if people believe a forest is haunted people will tent to keep out of it forming a natural barrier to hide your city...sure you got to kill a few people stupid/brave enough to ignore the "Haunted forest" signs but hey... what are a few lives in the greater picture?
A hollow Volcano that is only accessible through a hidden tunnel (perhaps an underwater tunnel). With artificial smoke being able to keep peeking eyes out.
The classic underground city is always an option. But kinda cliche (not that that is a bad thing)
On top off a steep mountain that is nearly impossible to climb (perhaps due to help from weather circumstances)
A hidden valley with hostile terrain surrounding it with a singular entrance point that can be traversed. Like the valley surrounded by giant mountains invested with monsters/animals or unbearable colds/storms but a small river going through a cave giving entrance to the area.