Ceiling
noun
The heaven well is the well that goes up into the heavens. This is above you, and it is a ceiling. Maybe the "well" part is referring to the chimney that goes up through the ceiling to the heavens when you look up in a super old house?
てんじょう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own. Be careful of the rendaku.
ある日天井に小さな穴が空いてるのを見つけたんですが、ストーカーがたまに屋根裏に上ってその穴から私のことを覗いていたことが後から判明したんです。
One day I saw a small hole in the ceiling, then later I found out that I had a stalker who would occasionally peep at me through the hole from my attic.