Castle Karstaag Caverns is a dungeon in Skyrim: Dragonborn
Inside the cavern, you can find something just up the tunnel into the entry chamber. You can see a number of bridges that will help you reach the final chamber of this.
Go over to the left-hand side of this pool that you find. You will find the corpse of Esmond Tyne. Turn to the South after that and go cross the water. This leads to a slope that goes up to the first bridge. You will also need to deal with a number of Rieklings that are patrolling the area.
Go up the slope and turn East to go across the bridge there. There is a small Riekling Campsite set up here. There is the odd Riekling here as well to deal with. Be sure to search the huts for some food and items.
Turn to the West, go back across the bridge and up the slope there. A few Rieklings will drop down from the ceiling in an attempt to ambush the Dragonborn. Follow this path to continue forward.
The tunnel will curve and lead you out to another chamber. There are more Rieklings standing guard around the area. A trio to the North gathered around a fire. Kill them off and then make your way to the West.
To the West you will find an Ice Door that you can open. Do so and on the other side you will find a water-filled cavern. Follow the wall to the right to find another Ice Door (Novice Lock) that leads to a small room with a skeleton and a coin purse. Press onward to the North to the next Ice Door.
This Northern Ice Door (Novice) has some loot and a random Skeleton. The door to the South of this one is an Apprentice difficulty lock. Inside it you will find a dead Riekling and some Riekling Spears.
Head to the East now and go through the door there. This will lead into a hallway. Go on up the slope there and into the next section.
Head across the walkway to the South. Go up the next slope into the next Riekling camp. There are Rieklings hiding the in the snow with at least one Rielking Scout patrolling and going into the watch tower.
Head to the South and then cross the bridge to the East. There is a tunnel there which will lead onward. There are a few more Scouts at the entrance of the tunnel. Deal with them then head on up the slope into the tunnel to the East.
Turn to the South out of this tunnel, going up the slope. Enter the tunnel there going to the West. Going to the North, downslope, leads to more Rieklings at a Watch Tower.
Follow the tunnel to the Ice Door. Open it up to find another Riekling camp. Fight on through to the West, across the next pair of bridges.
Go on up the slope to the West. This will lead you out to the Castle Karstaag Courtyard