The most important requirement for the admittance to a PhD at German universities in general is your university diploma, which has to be officially acknowledged as an equivalent to those German diplomas that are accepted as prerequisites to enter the PhD phase. German diplomas paving the way to the PhD track are Magister (old), Master (new), the “Staatsexamen” of education or the department of law, the “Diplom” as for German engineers. The regulations for the PhD, issued by each department of Freie Universität, all set up precise rules for admission.
If you want to check whether your diploma will be regarded as such an equivalent, you may consult the web portal "Anabin" maintained by the German Rectors’ Conference. The database of “Anabin” is quite comprehensive, and you will most probably find the desired information in it.
It is, however, nearly equally important that your field of studies matches the field your dissertation topic belongs to. While it is unlikely that a PhD candidate, coming from biochemistry, decides to do a PhD in law, it is not so easy to say why it should be impossible to write a dissertation thesis on French literature with a background in Classics. During the last years this matter has been made even more complicate by the institution of interdisciplinary MA-programs that do not facilitate the decision whom to turn to when you want to enroll for a PhD at another university. In cases of doubt you should contact the examination office or your desired academic advisor(s).