Which German city was the setting of the first series of the U.K. TV series "Auf Wiedersehen, Pet"?

"Auf Wiedersehen, Pet" is a British comedy-drama series about seven English migrant construction workers who leave the UK to search for employment overseas. In the first series, the men live and work on a building site in Düsseldorf. The series was created by Franc Roddam after an idea from Mick Connell, a bricklayer from Stockton-on-Tees, and written by Dick Clement and Ian La Frenais, who also wrote "The Likely Lads", "Whatever Happened to the Likely Lads"? and "Porridge".

The show starred Tim Healy, Kevin Whately, Jimmy Nail, Timothy Spall, Christopher Fairbank, Pat Roach and Gary Holton, with Noel Clarke replacing Holton for the third to fifth series. The series were broadcast on ITV in 1983–84 and 1986. A successful revival of the show saw two series and a Christmas special on BBC One in 2002 and 2004.

The first series, co-produced by Witzend Productions and Central Television for ITV in 1983, is the story of seven out-of-work construction workers from various parts of England who are forced to look for work in West Germany, although its initial emphasis is on three men from Newcastle upon Tyne making the journey to Germany, with the others being introduced along the way. (The title refers to their farewells to their wives and girlfriends - "Auf Wiedersehen" being German for "Farewell" or "Goodbye", or more literally "See you later", and "Pet" being a North-East English term of endearment).