The Batcave

 https://en.wikipedia.org/wiki/Batcave

The cool secret fortress under the mansion.

https://batman.fandom.com/wiki/Batcave

https://dc.fandom.com/wiki/Batcave

The Batcave is indeed like its own fortress.

https://www.cbr.com/16-things-you-never-knew-about-the-batcave

https://comicbook.com/news/batman-v-superman-did-we-see-superman-s-fortress-of-solitude-

Batcave 'Batman v Superman' Behind The Scenes https://www.youtube.com/watch?v=DdW4xcLCdcY

https://comicvine.gamespot.com/forums/batman-286/batcave-vs-fortress-of-solitude-which-one-has-more-650766/

https://www.fortressofsolitude.co.za/wayne-manor-vs-fortress-solitude

https://thedigitallabyrinth.blogspot.com/the_fortress_of_solitude