A Kingdom is a gathering of many people. It is ruled by a king and queen. There is normally 1 or more Castles in a kingdom.
For strategic reasons, kingdoms were normally built on hills. That way, they can see enemy attackers for miles.
Also, they normally had a strong rock wall surounding them. Apart from that, a moat was dug so that enemies could not reach the castle.
If many kingdoms are located in an area together, it can be called an empire.