Nikiou Fort
A giant fort in the Sapi-Res Nome area, with a captain who is formidable beyond his level. Scan thoroughly before launching your assault.
Note that the four treasures are nearly in a straight line running west to east- one in the building just inside the front gate, two in the central building, and one near the commander's quarters.
Wait until night, then approach from the west, taking out the lone tower guard (there is a secret entrance, but since you're going to be doing a lot of killing, its placement in the middle of camp isn't that helpful). From the tower, you have a vantage on all the nearby perimeter guards, who are too far apart to cover each other. Take them out (there's an arrow refill right here for when you're done, or you can sneak on over for the personal touch).
With the outer guards gone, enter the fortress itself from the southwest- this an ideal little corner to snipe from, lure enemies to, and stash bodies at.
There are a ton of guards here- the key is killing them quickly enough to avoid a mob. Do so by picking a choke point and sniping/stealth-attacking anyone who stops to look at all the other bodies. There's a parapet near the south west tower that's both a high-traffic site and yet visually obscured from many angles- it works nicely.
Don't worry about ammo- there's two refills in the tower, and plenty more nearby.
Once the area has been cleared a bit (of both guards and one captain, who often patrols this wall), drop down and assassinate the commander, who patrols near the front gate at ground level.
Kill any stragglers in the area (you do not want to leave anyone at your back here), then proceed into that treasure building near the front gate.
The first treasure here is in the second room on the ground level, in a small red box.
Snag it, then make for the middle treasure-holding building via the wooden balcony.
The next treasure is on a table inside.
Now is a good time to stop and do some more stealth-killing of the local patrols. The bodies can easily be dragged back in here, and multiple bodies can be hidden in the curtained closets.
When you've secured the area, ascend the building to find the third treasure in a large white chest.
You can now edge toward the final treasure, and get a vantage on the final captain.
For whatever reason, this guy is way tougher than the others, by an order of magnitude. The best way to prepare for this fight is take out any other nearby soldiers first, so you can focus on him alone. Also note that, if you want to rescue the civilians here, make it a priority- otherwise he'll probably kill them during the fight (his arrows are always on fire). When ready, start your assault with a sneak attack. It won't make a huge dent in his health, but at least it's a dent.
Trade a few licks, but when your health starts to drop, retreat up and around the nearby towers. Hide/evade while your health recovers, and snipe him when you see him.
Don't stick around, though- if you stay in the same spot too long, he'll ignite you with his own arrows. Instead keep kiting, sniping, and sneak attacking. When your adrenaline fills, use it.
This strategy will eventually take him down.
When he's dead, go to his quarters to claim the final treasure and complete the location.