Stämningsfull Julmarknad i Medeltida Stralsund is already pretty formal. Since it’s already descriptive, shortening it might lose some of the nuance. However, if you must have one shorter title, I suggest:
Julmarknad i Hansestaden Stralsund (Christmas Market in the Hanseatic City of Stralsund)
This retains a formal tone while referencing the historical significance of Stralsund which ties into the medieval theme.
