to get to fence area like you have to do what @Mytoastgotburned said.
to get nightmares achievement :
You have to go talk to Hawkfeather
-then find Beestripe and listen to her long backstory/until she has no new dialogue
-go back to Hawkfeather talk to her
-go to the area she pointed at there will be a new white arrow click it
-then keep walking in the same direction (left or ——> ) until you see two cats(or 1 depending on time of the day)
(REST OF THIS IS TO GET THE ACHIEVEMENT FEEL FREE TO IGNORE))
-then talk to the grey and white she-cat (Crystal until she has no new dialogue)
-walk back to Hawkfeather and talk to her
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Extra images for help:
Hawkfeather
Beestripe
White arrow after talking to Hawkfeather about the farm
The way to keep walking after click the white arrow
Crystal
