אחרים אומרים סוכה העשויה כשובך פסולה לפי שאין לה זויות
Aḥerim (R'Meir?) say: A Succah built in a circular shape like a dovecote is unfit, because it does not have corners. [and a permanent residence is one with corners]. (Succah 7b)
The Metivtah does not have an explanation, but I remember the Mishnah in Negayim 12 and Gemmorah in B"B 164b that says a round house IS a house:
תנו רבנן בית עגול דיגון טריגון פנטיגון אינו מטמא בנגעים טטריגון מטמא בנגעים
(Tosefta, Nega’im 6:3): A round house, or one that is shaped like a digon, i.e., it has two walls, one straight and one curved, or one that is shaped like a terigon, i.e., a triangle, or one that is shaped like a pentagon, does not become susceptible to the ritual impurity of leprous spots. If it is shaped like a tetrigon, i.e., a quadrilateral, it becomes susceptible to the ritual impurity of leprous spots.
Why, to Aherim, should a permanent residence, and therefore a Succah, have corners?