The Fortress of Solitude is a major location in the Arrowverse. It is Clark Kent's sanctuary in the Arctic, which holds information about his dead planet Krypton and allowed him to communicate with the A.I. of his late father. However, he began using his brother, Tal-Rho's fortress to consult with his mother's AI.
The first one was located in the Arctic and the second one was relocated in the middle of the ocean.