Where were castles usually built?

Castles were built next to a river or stream. This was so that the residents of the castle would get a fresh water supply. Also, some of the castles which have moats can fill it up with water. As well as being near water, a castle is usually next to a quarry. This is because when the castle was being built, the stone was painfully heavy and couldn't be carried a long distance. A castle would also need to be next to a village, town or city to keep an eye on. This was the whole point a castle was built!