Yes, participation is obligatory.
As a rule, in-house classes take place in Berlin on the Freie Universität Berlin campus at the end of September / beginning of October and at the end of February / beginning of March.
Usually, our students do not face any problems when applying for a visa. We are ready to help should any problem arise.
In Germany, in-house classes are recognized as "Bildungsurlaub". The conditions depend on the federal country you are working in. Please ask us if you wish to profit from such a regulation.