The Church of Peace (Friedenskirche) is located on the palace grounds of Sanssouci Park in Potsdam. It was built on the orders of King Frederick William IV of Prussia. He and his wife Elisabeth Ludovika of Bavaria are buried in the church itself while a mausoleum to the side houses the remains of Emperor Frederick III and his wife Victoria, Princess Royal. While the latter can not be accessed, due to the bad state the mausoleum is in, the former can be visited.
The church can be accessed free of charge and the entrance to the crypt is marked on the left-hand side of the church. The church is absolutely lovely and is not quite as busy as the rest of the park is. So if you need a breather from the grand palaces, do visit this church.