Siege Ladders: Ladders provide their crews with no cover whatsoever while approaching the Walls. Worse yet, the men using them will be severely disadvantaged trying to take a defended section of Wall, as they must attempt to climb and fight at the same time and due to their gaining the Walls a single man per ladder at a time. Nevertheless Ladders are cheap to build, much more difficult for defenders to destroy than Rams or Towers, and are capable of being used quickly if a section of Wall is undefended.I would also add that you can make your men run with ladders, which is impossible with siege towers or rams.