This is the Stadtschloss, the old Imperial government headquarters. It was heavily destroyed during the war and the East German Government blew up the remains:
The East German commies then built their Palast der Republic (Palace of the Republic) on the site. It was an ungodly, 1960s glass and steel monstrosity:
(Note, the domed building in the background is not the same dome, that's the cathedral further down the riverbank).
After the fall of communism, the Palast der Republic was bulldozed and the Germans are now rebuilding the castle from scratch. It's basically going to look just like the original on the outside, but will have a completely modern interior.
Google Earth view now. They're making pretty good progress:
https://www.google.com/maps/@52.5170426,13.4039207,3a,75y,284h,85.52t/data=!3m6!1e1!3m4!1s_6v7ZdvPGHTXyc1SOx9QJA!2e0!7i13312!8i6656?hl=en