I took advantage of this new area last night...in fact, I left Shurigen Castle to attempt it. Had to make a staircase going up...and jump like mad, but I did make it up there and was able to walk around and beat up two spiders and several priests.I looked for that entrance for that door...but never found it and assumed that it was a Easter Egg as well. Spent 2 hours overlaying maps and stuff to be sure that I hadn't missed a secret door.