A fortified town was built 7,000 years ago, at Armeggedon, or Meggido. Fortified towns are like castles, except that they don't have the huge spires in the middle, and the gaps between walls. If they had enough resources, they had two walls, but if they had a shortage, they would only build one. There were probably peasants, rich men, merchants and their children, and warriors in the fortified town.
After the fall of the Roman Empire, germanic tribes began to build stone fortifications. These were the beginning of medieval castles. In the next millenium, William the Conqueror built castles to protect the lands he had conquered like Warwick, London and Hastings. When he was in London, he built the Tower of London.
Castles are defensive structures. You will usually find that these castles have giant gates, giant towers and giant walls and probably buttresses which supported the walls. All the kings would have to protect their people if they wanted to stay in power because if they didn't they protect their people they weren't protecting themselves either and they wouldn't want to die.
The town, when under siege, probably didn't have a good water supply, so they probably had a good escape route to the river. This escape route would need protecting or (this way doesn't attract much attention) trees around it, to make the enemies less suspicious.
Castles were mainly defensive and fortified towns were defensive so that the villagers and townspeople didn't get hurt. Castles and fortified towns were usually on hills, because of the height advantage for the bowmen. And to throw rocks and boulders down the hill.
Castles were mainly defensive structures. Some were palaces that were fit for a King. Some castles are still intact even though they are thousands of years old.
