it was guarded by two demons, one on each side. the one on the left was mouthing "ah" and this one, on the right, was mouthing "nn". these are the first and last characters in the japanese syllabary, so it was kind of an alpha-omega...
so the hole in this pillar is the same size as the buddha's nostril. you're supposed to crawl through it and it will make you smarter. i could talk for an hour about the story behind it and the story of crawling through it, but i don't have room here.
incidentally, there is a pretty good replica of this temple that i happened across in hawaii. check out the hawaii album and imagine how surreal that was.