Since the early 1970s, he worked with artists like Udo Jürgens, Mary Roos, Heino, Rex Gildo, Michael Holm, Chris Roberts, Costa Cordalis, Mireille Mathieu, Peter Alexander, Roy Black, Karel Gott and Marianne Rosenberg.

In 1982, Siegel's and Bernd Meinunger's song Ein bißchen Frieden (A Little Peace), performed by Nicole won the Contest and became a hit in Europe.

He also composed 'C'était ma vie', performed by Lys Assia for representing Switzerland in the Eurovision Song Contest.

[3] Dschinghis Khan had been originally organized by Siegel and Dr. Bernd Meinunger to compete in the 1979 Eurovision Song Contest.

[4] After the success of their first hit, Dschinghis Khan, both men continued to write popular songs for this band, including "Moskau" and "Hadschi Halef Omar".