How do you complete the Ruins Of Belgrade?

To trigger The Ruins of Belgrade adventure you must enter Vienna by rail directly from Paris on the Orient Express. Passepartout will then be approached by a musician in the Austrian military who he can steal his flute from by plying the musician with wine. This is necessary as the flute will become very useful in the next stage of the journey when they have to board an Austrian military train that leaves for Istanbul.

After disembarking the train in Belgrade Fileas Fogg and Passepartout must explore the ruined city at night so they encounter a partisan so they can give him the stolen flute to help him fight against the armies of mechanical men. Decline the mans offer of a ride in a cart to Sohia and continue on the train to Istanbul. At Istanbul Fileas Fogg and Passepartout will witness the foiling of the invasion and will be able to proceed with their journey.

Alternately, If you did not steal the flute from the musician you will need to travel to Budapest by car, then Bucharest by train. At Bucharest you will need to deliver the glass cube of the dead man to the artificers guild. If you do this, you will not encounter Herr Danzer in Vienna.