Start exploring the new area of the Mountain Village, heading for the cave exit. Soon you'll reach a new base camp (Mountain Descent); rest to unlock the fifth Lara's Journal. Approach the nearby fork in the road; this can only mean that there are collectibles hidden in the area. If you want to keep to the current objective, stay on the main path and leap over to the destroyed drawbridge.
Be cautious, you'll run into three guards shortly. Take cover and get ready to attack. Considering that the enemies won't notice you right at the beginning, you can use the bow again, though obviously the submachine gun would be a safer choice.
Jump over a small gap to search their bodies. Afterwards, stand at the above edge and make a rope bridge. Slide down and blast the entrance to the building with the shotgun. Go inside to trigger a cut-scene.