It's just a farming spot for earn XP fast for SKILL points or for gain Attack Power for an easier game. I tried all other route/way, but this is the fastest (most XP/min) and easiest (no fight). It's 26-28 secs for a round (and ~8500 XP on NG+2 in the video). It's before end game and you need the OGRE ALIVE in Ashina Castle. You have to whistle to distract the two "red hat guy" below, after run back and You have to look down to the Ogre to make him mad and unchained. Then run to the idol (and if necessary wait for the Ogre to kill the two guys [in the video 2x2.702 exp points])
I used this to get +10 Attack Power (+50 points!!!) from 40 - > 50. I couldn't kill Isshin Sword in NG+2 (3rd endgame) on AP 35-40 even it was the 3rd time for me! When I got level 50 AP I killed on 2nd try. The extra damage wasn't much, but the posture damage was great, his posture damage grown much more faster! Just for your information the last level for me was 260k.
I can get now 8.345 XP at Ashina Outskirts, if kill only the first 5 enemy (tower/under tower, at gate, and 2 walking on the stairs up). It's around 37-40 secs, if you are fast, but 5 kills and maybe make a mistake. If you go further you lose points/time, because even you can get around 2x points, the time is around 2 mins and 15-30 secs with the teleport time, while you can do 3 short runs for 3x8.345 XP per 2 mins.
With this I get 8.560 XP per 27-30 secs. No fight, just 1 stealth kill, so easier and faster.
So 8.345 XP & 40 secs VS 8.560 XP & 30 secs = 751k VS 1.027k per hour
Here is a video of the Ashina Outskirts run:
PS: First I collected 25 points for my last 25 skills for the achievement. Used the save method. Spent all points, get the achievement and immediately recover all 25 points, then I spent to 5 AP and I started to get my last +10 AP from 40 > 50. Attack Power (damage gain) for each level:
PS2: My best skills for bosses Shadowrush/Shadowfall and Ichimonji Double. I wish I could regain my wasted points from useless skills.