DLC: How do you access north and south areas of map?

[Comrade Maethor]

I've tried my best going blind but I genuinely have no clue how to access the areas south of Gravesite Plain and north of the Shadow Keep. There's an obvious door that leads out from the Storehouse but there isn't even an interaction for it. Another way out by the eastern side of the castle also looks like it only leads out to a restricted area. As for the south, I rode along the whole rim of Gravesite Plain and couldn't find a way down. The northern area also doesn't look like it would wind south but idk.