Murder Holes is a technology in Age of Empires II that can be researched at the University. Once researched, it removes the minimum range from defensive structures. It is the only non-unique technology to cost stone. Murder Holes almost always worth getting since it allows Castles and Towers to stand alone and defend themselves against infantry and cavalry. Rams, however, remain a threat due to their practical immunity to arrow fire. So nevertheless, constant attention of melee units is advised in order to keep defensive structures up.