A keep is a square structure that could easily defend. The keep was also thre tallest and strongest structure in the castle. It was used as the last line of defense during a attack or siege.I hope...
Square keep castles changed to concentric castles because castles got upgraded as we evolved, so did ideas for castles with more defence, space and attack power.
A Square Keep castle usually sits on a hill. It only has 1 wall of defence, whereas a concentric castle usually has about 3 thick walls. Concentric's also have a keep in the middle that has a wall...