From: Rabbi Moshe Sofer
To: Rabbi Yechezkel
R' Yechezkel proposed a proof from Mishnah Ohalos for the Rambam's ruling that a non-square room with sufficient area to form a four-by-four amos square is obligated in mezuzah. The Chasam Sofer rejects this proof, arguing the Rosh can explain the Mishnah differently, and resolves an internal tension within the Rambam's rulings by noting that mezuzah has a uniquely broad scope because the Torah intends God's commandments to be visible at every entrance.
Source: Shu"t Chasam Sofer YD §280