Recidence Palace has been declared a UNESCO World Heritage Site. The stairs next to the entrance is really spectacular. The entry itself is worth a visit. The photographs there explain the reconstruction of the palace after WWII. There are spectacular gardens, which incidentally are free.
Festung Marienberg is known in German as the impressive Marienberg Fortress. This fortress, which is the largest I´ve seen in my life, is located in Wurzburg in the middle of Romantic Road in Germany and can be seen from anywhere in the city. It´s nice to visit in winter but it´s even better during the spring and summer. I also highly recommend accessing the fortress on foot and walking among the vines leading up to it. It´s free to enter but I recommend the guided tour (in German or English), because it shows you incredible parts of the fortress. The wall, the moat, the gardens and the views of Würzburg cannot be missed.