Apollonia Roman Barracks (Location)
A Roman compound in Kyrenaika. You'll raid it during the 'Dead in the Water' side quest.
A scan will show that all the interesting stuff here – the treasure, the captain (and the quest objective), are on that same southern wall.
Climb up from the outside, and conscientiously trap the brazier.
You can see the top floor of the captain's office from here.
Climb down the ladder and off the captain if he's there. If not (or after you've killed him), descend the stairs.
This will take you to the treasure, in a small red chest.
Ambush the captain on your way back up, if he's not already dead, to complete the location.