Related על סוכה 8:16

But the sages say: Two [walls] by regulation, and the third even only a handbreadth high. R. Shimon says: The third by regulation, and the fourth even only a handbreadth high. R. Shimon son of Elazar says in the name of R. Meir: If there are two [walls] by human hands and the other one is a tree, then it is fit and one may climb up to it [to the sukkah] on a holiday.