Not really a cathedral
Despite its name, it has never been the seat of a bishop. It's the Protestant Supreme Parish and Collegiate Church, built as the court church of the Hohenzollern dynasty.
Wilhelm II wanted a Protestant answer to St. Peter's in Rome, right next to the royal palace. Here's what you're looking at on Museum Island, and what to listen for when you get there.

Despite its name, it has never been the seat of a bishop. It's the Protestant Supreme Parish and Collegiate Church, built as the court church of the Hohenzollern dynasty.
In May 1944, a fire burned the lantern on top of the dome and it crashed into the church. In East Berlin, restoration only began in 1975, and the church reopened in 1993.
The Hohenzollern Crypt holds around 90 coffins of the dynasty, including Frederick I, the first King of Prussia, and his wife Sophie Charlotte.
No, it's Protestant, and despite the name it has never been a bishop's seat.
Yes, 270 steps lead to a walkway around the dome.
Between 1894 and 1905, for Emperor Wilhelm II, designed by Julius Raschdorff.