Stone Castles
Timber castles were easy but if you wanted a more permanant home they had to build with stone.
Stone castles were much stronger than timber castles, not only because it was made with stone, but also because of the way they were made. Stone castles were also heavier than castles made with timber. When stone was used, the keep was too heavy to be built on a motte, the mound of dirt that the earlier timber castles were made on. A moat surrounded the stone castle making it harder to attack.
Some stone castles were concentric; a concentric castle had two sets of walls so it was very strong. Strong enough that it didn't need a keep so it had more room in the castle. By the 12th century most new castles were made with stone. They were made to resist attacks and, or sieges. Stone castles would withstand almost any thing, until gunpowder was brought over from China in the 14th century.