הלכה על עירובין 101:20

2. What is the private domain? A place surrounded by walls that are ten tefachim that is four by four, or a pit that is ten tefachim deep and four by four, or a hill that is ten tall and four by four. Rem"a: Some say that the diagonal also must be four tefachim, like we will explain below in chapter 349. (Hagahos Ashiri; Tosefos)