Siman 688
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א כְּרַכִּים הַמֻּקָּפִים חוֹמָה מִימוֹת יְהוֹשֻׁעַ בִּן נוּן, אֲפִלּוּ אֵינָן מֻקָּפִין עַכְשָׁו, קוֹרִין בְּט"ו, אֲפִלּוּ אִם הֵם בְּחוּצָה לָאָרֶץ, וַאֲפִלּוּ אֵין בָּהֶם עֲשָׂרָה בַּטְלָנִים (פי' בַּטְלָנִים מִמְּלַאכְתָּם וְעוֹסְקִים בְּצָרְכֵי צִבּוּר); וְהוּא שֶׁהֻקַּף וְאַחַר כָּךְ יָשַׁב, אוֹ שֶׁיָּשַׁב תְּחִלָּה עַל דַּעַת לְהַקִּיפוֹ אַחַר כָּךְ, לַאֲפוּקֵי כְּשֶׁנּוֹדַע שֶׁיָּשַׁב תְּחִלָּה עַל דַּעַת שֶׁלֹּא לְהַקִּיפוֹ אֲבָל בִּסְתָמָא הֻקְּפָה וּלְבַסּוֹף יָשְׁבָה (רַ"ן).
1 The law for cities which were surrounded by a wall in the days of Yehoshua Bin Nun, 8 Seifim: Cities that had a wall surrounding it at the time of Yehoshua Bin Nun, even if they currently do not have a wall surrounding them, read the Megillah on the 15th of Adar, even if they are outside the Land of Israel, and even if they do not have ten Batlanim (explanation: people who are unemployed and who busy themselves with the needs of the congregation). The above refers to [cities that are] surrounded, and afterwards were settled, or were settled at first with the plan to build a wall surrounding them afterwards - this comes to exclude [a city in which] it becomes known that it was settled with the original intent not to surround it [with a wall] [but a plain case scenario can be assumed to [involve a city] surrounded and then settled] [the R"an].
ב וְכֵן הַכְּפָרִים הַנִּרְאִים עִמָּהֶם, אֲפִלּוּ אֵינָם סְמוּכִים, כְּגוֹן שֶׁהֵם בָּהָר; אוֹ שֶׁסְמוּכִים לָהֶם, אֲפִלּוּ אֵינָם נִרְאִים עִמָּהֶם, כְּגוֹן שֶׁהֵם בָּעֵמֶק, וּבִלְבַד שֶׁלֹּא יִהְיוּ רְחוֹקִים יוֹתֵר מִמִּיל. וּבְשׁוּשָׁן אַף עַל פִּי שֶׁאֵינָהּ מֻקֶּפֶת חוֹמָה מִימוֹת יְהוֹשֻׁעַ, קוֹרִין בְּט"ו הוֹאִיל וְנַעֲשָׂה בּוֹ הַנֵּס.
2 And likewise the villages which are visible with them [the "walled" cities], even though they are not [directly] adjacent to them, such as when they are [together] on a mountain. Or they are adjacent to them even though they are not visible to them, such as when they are in a valley, provided they are not farther than a mil. And in Shushan, even though it was not surrounded by a wall from the days of Joshua, they read on the 15th [of Adar] because this is where the miracle occurred.
ג כְּפָרִים וַעֲיָרוֹת גְּדוֹלוֹת, וּכְרַכִּים שֶׁאֵינָם מֻקָּפִים חוֹמָה מִימוֹת יְהוֹשֻׁעַ בִּן נוּן קוֹרִין בְּי"ד.
3 Villages, large towns and cities that did not have a wall at the time of Joshua the son of Nun read on the 14th.
ד כְּרַךְ שֶׁהוּא סָפֵק אִם הֻקַּף בִּימֵי יְהוֹשֻׁעַ אִם לָאו, קוֹרִין בְּי"ד וּבְט"ו וּבְלֵילֵיהוֹן, וְלֹא יְבָרֵךְ כִּי אִם בְּי"ד שֶׁהוּא זְמַן קְרִיאָה לְרֹב הָעוֹלָם.
4 A city regarding which there is a doubt whether it was surrounded by a wall during the time of Joshua or not, they read on the 14th and the 15th as well as on their evenings. One makes a blessing only on the 14th, which is the time of reading for the majority of the world.
ה בֶּן עִיר שֶׁהָלַךְ לַכְּרַךְ, אוֹ בֶּן כְּרַךְ שֶׁהָלַךְ לָעִיר, אִם הָיָה דַּעְתּוֹ לַחֲזֹר לִמְקוֹמוֹ בִּזְמַן קְרִיאָה (שֶׁל י"ד) וְנִתְעַכֵּב וְלֹא חָזַר, קוֹרֵא בִּמְקוֹמוֹ; וְאִם לֹא הָיָה בְּדַעְתּוֹ לַחֲזֹר אֶלָּא לְאַחַר זְמַן הַקְּרִיאָה, קוֹרֵא עִם אַנְשֵׁי הַמָּקוֹם שֶׁהוּא שָׁם. הַגָּה: וְאִם הוּא בַּמִּדְבָּר אוֹ בַּסְּפִינָה, קוֹרֵא בְּיוֹם י"ד כְּמוֹ רֹב הָעוֹלָם (כָּל בּוֹ).
5 A resident of a town that went to a city or a resident of a city that went to a town - if it was his intention to return to his place at the time of the reading and he was delayed and did not return [to his usual place], reads according to his [usual] location; and if it was his intention to return only after the time of the reading, he reads with the people of the place where he is currently. Rema: And if one is in the desert or on a boat, one reads on the 14th like the majority of the world. [Kol Bo]
ו יוֹם חֲמִשָּׁה עָשָׂר שֶׁחָל לִהְיוֹת בְּשַׁבָּת אֵין קוֹרִין הַמְּגִלָּה בְּשַׁבָּת, אֶלָּא מַקְדִּימִים לִקְרוֹתָהּ בְּעֶרֶב שַׁבָּת, וְגוֹבִים מָעוֹת מַתְּנוֹת עֲנִיִּים וּמְחַלְּקִים אוֹתָם בּוֹ בַּיּוֹם; וּבְיוֹם שַׁבָּת מוֹצִיאִים שְׁנֵי סְפָרִים וּבַשֵּׁנִי קוֹרִין: וַיָּבֹא עֲמָלֵק; וְאוֹמְרִים: עַל הַנִּסִים; וְאֵין עוֹשִׂים סְעֻדַּת פּוּרִים עַד יוֹם אֶחָד בְּשַׁבָּת.
6 If the 15th falls on Shabbat, we do not read the Megillah on Shabbat. Rather, they advance the reading to Friday and collect money for gifts to the poor and distribute them on that day [Friday]. And on Shabbat, they take out two Torah scrolls, and in the second they read "And Amalek came" and say Al HaNisim, and do not make a Purim feast until Sunday.
ז הַמְפָרֵשׁ בַּיָּם וְהַיּוֹצֵא בְּשַׁיָּרָא וְאֵינוֹ מוֹצֵא מְגִלָּה לְהוֹלִיךְ עִמּוֹ, יִקְרָאֶנָּה בְּי"ג אוֹ בְּי"ב אוֹ בְּאֶחָד עָשָׂר, בְּלֹא בְּרָכָה; וְאִם אִי אֶפְשָׁר לְהַמְתִּין עַד יָמִים הַלָּלוּ, יֵשׁ אוֹמְרִים שֶׁקּוֹרֵא אֲפִלּוּ מִתְּחִלַּת הַחֹדֶשׁ. הַגָּה: וְהָכֵי נָהוּג; מִיהוּ אִם נִזְדַּמֵּן לוֹ אַחַר כָּךְ מְגִלָּה, חוֹזֵר וְקוֹרֵא אוֹתָהּ בְּיוֹם י"ד, אֲפִלּוּ קְרָאָהּ תְּחִלָּה בְּיוֹם י"ג מִכָּל מָקוֹם קָרָא אוֹתָהּ שֶׁלֹּא בִּזְמַנָּהּ (כָּל בּוֹ וּבֵית יוֹסֵף).
7 If one will be sailing at sea or going out in a caravan and does not find a Megillah to take with him, he is to read on the 13th or the 12th or the 11th without a blessing. And if he cannot wait until those days [before travelling], there are those who say that he is to read even starting from the beginning of the month [of Adar]. Rema: And this is our custom. However, if by chance he comes upon a Megillah afterwards, he goes back and reads it on the 14th, even if he read it initially on the 13th, [since] in any event he had read it not in its appointed time. [Kol Bo; Beis Yosef]
ח בֶּן עִיר שֶׁהָיָה בַּסְפִינָה אוֹ בַּדֶּרֶךְ וְלֹא הָיָה בְּיָדוֹ מְגִלָּה, וְאַחַר כָּךְ נִזְדַּמְּנָה לוֹ בְּט"ו, קוֹרֵא אוֹתָהּ בְּט"ו.
8 A resident of a town that was on a ship or on the road and did not have a Megillah with him, and then afterwards came upon one on the 15th, should read it on the 15th.