Walk up the left side until you see the top of the left guard's shield. Aim your bow at the right guard's chest. Fire three poison arrows to poison him. Be patient and he will die. If you do not have poison arrows, use what you have until he is defeated. Then, go down and walk up the right side hugging the rail step onto the platform. The guard will look at you, walk to your right, and turn back around. Then do the same thing you did to the first guard.