Hell Castle was Dr. Hell's new base after he and his lieutenants were forced to retreat off Bardos. It's first full appearance was in episode 60 of the anime. Originally part of the Izu Islands, Dr. Hell had the Mechanical Beasts, Giant F3 and Titan G9 remodel the island in quite literally his image.
Hell Castle's most defining feature is Dr. Hell's face carved onto the front of the island. It features most of the same facilities as Bardos including a docking area for the Gool just at the top of the island, a station for the Bood on the inside of the island, and an array of missiles for defense.
After being forced off of Bardos, Dr. Hell used Hell Castle as his new base of operations. It is here Dr. Hell and his lieutenants meet Archduke Gorgon as well as the Ghost Mechanical Beasts. When the Mazinger Z and its allies make a final assault, Dr. Hell sends out the Haribiyun V6 and Saiga O3 until they were destroyed and Dr. Hell had the island self-destruct thinking it wasn't a home anymore.
In the manga, as the Mazinger Z and Venus A went towards the island, Dr. Hell unleashed the Hell King Gordon, a massive Mechanical Beast made in the center of the island. After a difficult battle, Koji managed to get the Mazinger Z make a direct attack onto the Gordon destroying it, Dr. Hell, and what remained of Hell Castle.