A lot has happened since then: the city was once part of Germany, then part of the Danish kingdom. The teacher training college became the European University and SG Flensburg-Handewitt became both German champions and Champions League winners. You can find a detailed overview of Flensburg's city history here.
Another tip: The city archive in Flensburg's town hall is a real treasure trove for history buffs or those who simply want to find out more about Flensburg's eventful past. You are cordially invited to drop by and browse through the historical documents, newspapers and photos.