This is the 7588th most frequent Russian word.
дюссельдорфе
"Дюссельдорфе" is the prepositional case of "Дюссельдорф," meaning "in Düsseldorf," the name of a city in Germany.
Here 'в Дюссельдорфе' indicates the location where an action (being) occurred, using the prepositional case.
Я был в Дюссельдорфе прошлым летом.
I was in Düsseldorf last summer.
In this sentence, 'в Дюссельдорфе' again uses the prepositional case to describe the location.
В Дюссельдорфе проходит знаменитая ярмарка.
A famous fair takes place in Düsseldorf.
Here 'в Дюссельдорфе' specifies where the action of staying took place, emphasizing the prepositional case for location.