what does it mean to occupy an area does it mean i need to keep a unit back like risk ? or can a single Stronghold keep the land and if so does a City do the same ?

Ripley... said:

Page 15 has a lot of definitions. Among them, it talks about what it means to control an area: basically, you control an area if you have any units there, OR a stronghold. The main exception is Home Realm areas - you control your home realm areas even if you have no units there (unless an enemy controls the area with units or a stronghold; see the FAQ for the Stronghold clarification).

A city on a tile does not control it, afaik. The city itself must be controlled by leaving units there, so it certainly can't control a hex for you if left alone. Strongholds DO control a hex even without units, but if memory serves you can't build a stronghold on a hex with a city. Is that just my imagination?

Also, heroes do not control hexes. Seems obvious, but I've had to remind myself on more than one occasion that having a hero in the city hex doesn't mean I own the city.

Yes, cities prevent strongholds from being built

but you may destroy it via the tactic card (whatever it is called) so you can build your stronghold there

(this question is also answered in sigmars compiled corey rulings)