I have never lived in one, you understand, but I hear in medieval times, castles came equipped with keeps. A castle keep was the place in the castle where important documents were kept. It was the place the king would store his crown, the royal jewels, and his kingly robes. Most of the time, the keep of the castle also had living quarters so the king and his family could be kept safe when the castle was under attack.
About the time of William the Conqueror of England, the architecture of the castle keep was changed. It seems those square castle keeps in the middle of the outside castle wall were good targets for the battering rams of the king's enemies. Hence, the castle keeps became round and were placed in the middle of the castle, keeping the keep more protected.
When the king was attacked, the keep was a good place to keep the king. In the keep, it was easier to protect and defend the king, keeping the king's family safe. The keep was a good place to be kept!
There is no royal blood in my genealogy. No kings or rulers grace my ancestral chart. Neither my grandparents, nor their grandparents, nor even my grandparents' grandparents lived in a castle. When times of conflict came, my relatives were not kept in the keep. They were the ones on the front lines, keeping their country safe.
Yet, still, today I can claim to be a kept woman. I'm a princess, even, living in the castle keep. My Father, the King, keeps me in His protection and care. He speaks over me the words He spoke to the Israelites: "The Lord bless you and keep you; the Lord make His face to shine upon you and be gracious to you; the Lord turn his face toward you and give you peace," (Numbers 6:24-26).