Vilicus Castra
An enemy encampment in northwest Arsinoe. You'll do a number on the place during 'The Setting Sun' story quest. The open plan of the fort makes it easy to infiltrate, but the northeast corner is particularly vulnerable.
Apart from the easy access to watchtower and rooftops, there's a gap in the wall right where the fort commander goes for his pee breaks.
Assassinate him first, then sneak or snipe anyone in a watchtower.
This should give you some room to sneak or snipe the base captain, in the northwest corner of the fort.
Treasure wise, one is in a red chest, in a tent in the southern half of the area.
The other is a box of carbon crystals in the pretty building in the northeast corner.
Accomplishing all this completes the location.
You can also free prisoners here for XP and achievement progress.