Belvedere on the Pfingstberg

Since the reopening of the Belvedere Palace in 2001, visitors have enjoyed a unique view of Potsdam and the nearby cultural landscape as far as Berlin.
The opening times of the Belvedere:
April to October: daily 10 a.m. to 6 p.m.
March and November: Saturdays and Sundays 10 a.m. to 4 p.m.

It is not so much the castle that is worth seeing or what is left of it, but rather the view from the tower.
The entrance fees:
Regularly € 4.50
Reduced € 3.50
Children / adolescents 6 to 16 years € 2.00
Children up to 6 years free
from 70% disability free
Family ticket € 12

More information: pfingstberg.de

The "Belvedere auf dem Pfingstberg" is a palace to the north of the New Garden belonging to the Potsdam Palaces and Gardens ensemble and is located on the Pfingstberg, the 76 m highest point in Potsdam west of the Havel. It was built under Friedrich Wilhelm IV because of the beautiful view and is only part of what was originally a much more extensive building project. The twin tower complex, modeled on the Italian Renaissance, experienced two construction phases, the first of which was from 1847 to 1852 and the second of which followed after a long break from 1860 to 1863. The castle is the only monument in the world heritage ensemble that is operated by a private association that was founded in GDR times to preserve the decaying building.

Source: de.wikipedia.org/wiki/Belvedere_auf_dem_Pfingstberg