From: Rabbi Moshe Sofer
To: Unknown recipient
The Chasam Sofer analyzes whether the Megillah may be handled on Shabbos Erev Purim, whether Purim night carries Yom Tov status, and the rationale for requiring walls from the time of Yehoshua bin Nun.
Source: Shu"t Chasam Sofer OC §195