It is a huge castle found in Zenith. It has gargoyles in it. The structure is old and most of it is destroyed, probably by Frankenstein's Monster. In it, also lives the Bride of Frankenstein. According to Stormy, the Bride of Frankenstein, electricity brought Frankenstein's Monster to life. This must mean that the place was hit by powerful lightning bolts, seeing that there are no signs of life around it.
In "A Monster's Crush", after Selina brought Frankenstein's Monster to life, she send him to Zenith to cause chaos and destroy the Winx. As they fight, Tecna, using Zenith's electrical energy, attacks it and makes it fall madly in love with her. When Frankenstein's Monster tightly hugged her, Selina put it, along with Tecna, in the Castle of Frankenstein's Monster. She also put Stormy in there as the Bride of Frankenstein to make sure that Tecna stays there too long and get stuck in it forever. When the Winx enter the Castle of Frankenstein's Monster and fought Stormy, Tecna empowers Stormy to make her electricity stronger and Frankenstein's Monster fell in love with Stormy. The Winx, then, escaped and so did Stormy.