The Fortress of Solitude is Superman's arctic sanctuary.
Clark invited Kara to the Fortress of Solitude several times, though Kara declined as she thought it would remind her too much of Krypton.
It was mentioned by Alex to Kara that Superman uses the fortress as a base where he can communicate with his Kryptonian ancestors.
Kara along with James visited the Fortress to gain information regarding Indigo.
Kara later visited the Fortress seeking out Kal-El's whereabouts and then the purpose of Myriad.
- The Fortress of Solitude in the comics, is Superman's sanctuary located in the Arctic. It first appeared in Superman #17 (July, 1942), created by Jerry Siegel and John Sikela.