As the new Black Hand Listener, Sithis and the Night Mother have blessed you. Since the other Listener is dead, his castle is yours. The Night Mother provided it with sheeps and everything you need to survive, plus the grass.
Sithis, on the other hand, provided it with weapons, human guards, magical gates and doors, and since you are an exceptional person, the greatest Listener that ever lived, he decided to open the long lost Black Hand Caverns.
It was a myth, but seems they are running all under the Black Hand Castle.
What awaits in those lost caverns? Sithis did not tell you. It's up to you. He didnt tell either the emplacement of the entry, which is hidden and does not appear on any map.
Great rewards await, but great dangers too.

Other than that, you can pray at the Altar of Darkness to restore your abilities, in the Chapel of Night.
The Guard stations contains everything you need to survive. Food, weapons, guards...

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Basically, it's in the Jerall mountains, and there's an Oblivion Gate nearby.
For those who have Frostcrag Spire, its just near that.
Your guards respawn, so no worry.

There is NO WAY (almost) to get inside the perimeter but to get through the gate. I tested it myself, the house is on top of a hill, it's one of the highest points of the Jerall Mountains, and you can see Cheydinhal and the Imperial City and Bruma (behind the house) from up there.
Really good view.

There is a key to the house, chapel, and everything else. You can get in the perimeter for now, but you can't enter anything until you are the Listener and that you have the key for it. (QUEST IN PROGRESS)

The guards will all have leveled weapons.
Basically, it's it.
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All other details in the readme, included in the zip.