There is a hornet's nest in a tree close to where the guard walks. Shoot it when the guard is directly under it. He will be attacked by the hornets and run screaming across the bridge. He will actually affect most of the guards on the other side of the bridge but you will have to be fast to get to the other side before they return.

If you are spotted on the bridge, cut the ropes with your knife and the guards may start falling off the unstable bridge as you cross (just be careful not to fall yourself )

When you reach the other side, there is a trail on the far left of the screen that will take you to a cliff side under the bridge that has some extra goodies to help you out.