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It's part of a achievement/quest. The one called Nightmares gives you access to it. Sorry, but idk how to get back there again. 

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

——————-

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