City = مدينة

Cities = can be both مدن or مدائن

but مدن is preferable. مدائن is releatively old and is bairly used these days. 

both are the plural of مدينة 

http://library.islamweb.net/newlibrary/display_book.php?idfrom=7777&idto=7777&bk_no=122&ID=7789

نفس الشيء
مثلا سنوات وسنين
او مشاكل ومشكلات
او اغنيات واغاني
الخ

المدينة تجمع على مدائِن، مدْن، و مُدُن

Madinah is singular, it's plural can be either Mada'en, Mudn, or Mudun

:) you can try using baheth.info, a collective of Arabic dictionaries (only Arabic-Arabic) but it's very detailed so you should be  B2 or higher. 

Thank you Emir but the two words I mean are مدن / مدائن