Complex sentence: Only one portal section, from the balcony of which Liebknecht had declared the German Socialist Republic was preserved, and it was later added to the Council of State building (Staatsratsgebäude) where it formed the main entrance.
Split into simpler sentences: Only one portal section was preserved, a portal from the balcony of which Karl Liebknecht had declared the German Socialist Republic. It was later added to the Council of State building (Staatsratsgebäude) where it formed the main entrance.