Megillah 19: Who, How, When, and Why We Read Megillah

2 mishnayot - when do mitzvot need to take place, from Megillah to a whole host of other mitzvot? What about a walled-city resident who goes to an unwalled town, or the reverse? Which leads into the nature of what we do that season, remembering Amalek and so on. Also, who can read the Megillah, and who cannot. And why "deaf" means something different from the expected "deaf-mute" who so often appears in rabbinic literature.