Medieval spiral staircases in castles are running clockwise. This is because all knights used to be right-handed. When the intruding army would climb the stairs they would not be able to use their right hand. When the right hand was holding the sword, this would be difficult to climb the stairs. Left-handed knights would have had no troubles, except left-handed people could never become knights. This is because it was assumed that left-handed people were descendants of the devil.