Simplify the following text for a Grade 9 native English-speaking person.
Original: The city was first mentioned in written sources in 1323 , in letters of Grand Duke Gediminas that were sent to German cities and invited Germans and members of the Jewish community to settle in the capital city .
Simplified: The city was first mentioned in written sources in 1323 as the capital city of the Grand Duchy of Lithuania in the letters of Gediminas .