Northwatch Keep is a dungeon in The Elder Scrolls V: Skyrim
Haaginar - Northeastern area of Skyrim
The outside is protectde by a number of Northwatch Guards. They are backed up at least 2 Northwatch Archers. Take your time to either slip in or deal with them. It is impossible to just talk your way through them. The moment you attack one of the guards, they will all turn hostile on you. Defeat them all. After that find the door leading inside the keep itself.
Inside, you will find find another Northwatch Guard with a Northwatch Mage helping him out. Take them out and then follow the passage leading downward.
After the landing, slow. There is another Northwatch Archer ahead patrolling. Attacking them will prompt 2 more Northwatch Guards from the next room to come rushing in to join the fight. Take them all out then head down the stairs into a small tavern.
In the bar of the tavern, look on the far right-hand side. On the bottom right of the shelving you will find the One-Handed Weapons skill book 2920, Morning Star, v1. Read it, grab it and then head forward into the next large room.
In this room you have at least 2 Northwatch Mages, 2 Northwatch Guards and 2 Northwatch Archers. Defeat them all then take a little time to search the room. There is a lot to be found in here. Take your time to search, especially to find The Legendary Sancre Tor which sits in the room on the shelf across from the door in.
Head up the stairs and over to the right. You will come to an intersection. Head over to the left. This will lead to the Northwatch Interrogator who is holding Thorald Gray-Mane. Kill the Interrogator to get the Northwatch Keep Key. This will let you easily leave (as the exit door is a Master Level lock). You can also find Northwatch Prisoners.
Turn to the North now. You will find a small set of 4 cells. You can pick them all (All 4 are Master Level locks) or just use the levers inside the room on the Western side. From there, just go up the stairs and use the Key to leave this area.