Behind the iron gate is a giant tree, where four lanterns can be lighted. (This is a side quest that can be left unchecked.)
Play the flute to the stone tablet near the tree after lighting the four lanterns.
Pay attention to the nearby sign pointing at north.
Play the flute to the four facets of the tablet according to the four directions “North, east, west and south”. Then, a corpse will fall from the tree, from whom a map can be obtained. The location of the tree can be found on the map. Go to the destroyed bridge above the river and process along river to a grave, where the reward of this side quest is hidden. From there, a passage in the north spreads across the river.
Go to the grave as shown on map and take the cassette on the gravestone.
Then, the mailbox can be interacted with to acquire additional files. There are six mailboxes within the current chapter.
Return to the giant tree and exit the iron gate. There is a mailbox at the initial crossroad.
