Balagrae Roman Barracks (Location)
The Roman garrison of Balagrae, in the Green Mountains. This is the site of the Shadows of Apollo side quest.
Scans show a well-populated but segmented fort- with the captain lingering in his office in the center building or the campfire east. The treasure floats below on some subterranean level.
Come in from the rocky wall on the north. This will give you an easy shot at clearing the battlements here.
You can then use the walls, catwalk, or rope to get a vantage on the captain.
Getting to the treasure can be a little confusing, because the underground entrance isn't on the lowest floor of the complex. Instead, enter the door nearest the north east privy.
Take the narrow stairs down.
Now you can loot the treasure from the red chest in the corner.
This completes the location.
Note that you are in exactly the right spot to launch the Shadows of Apollo side quest by freeing the prisoner here.