Complex sentence: It was used as a stronghold for regime-leader Superman's enemies, since Superman is vulnerable to magic, and after the end of his regime, it was also used as a prison for regime members suggested by the public to have a death penalty.
Split into simpler sentences: It was used as a stronghold for regime-leader Superman's enemies, since Superman is vulnerable to magic. Also, at the end of his regime, it was used as a prison for the remaining regime forces despite the public demanding them to have a death penalty.