Cardiff Hill plays an important role in Tom Sawyer’s world.
There is more than one hill in the book. Early on in the book, Tom is trying hard to concentrate in school and looking out at Cardiff Hill. He describes it in detail.
Away off in the flaming sunshine, Cardiff Hill lifted its soft green sides through a shimmering veil of heat, tinted with the purple of distance; a few birds floated on lazy wing high in the air; no other living thing was visible but some cows, and they were asleep. (chapter 7, p.38 enotes etext)
Tom and Huck visit the hill often, but since it is behind the widow’s house and technically on her land, they are careful. This is where they play Robin Hood.
There is also a cemetery on a hill, and the hill three miles from town with the caves in it.