The Fortress of Solidarity was a fortress located inside a tesseract outside of normal space-time, used by various heirs to Superman's legacy in the future. The supermen and women (and others) who use the fortress come from different eras across time, and all work together inside the Fortress.

Unlike the Fortress of Solitude (of which many of the future Supermen have their own version), the Fortress of Solidarity is a place for them to be together, rather than alone. ("Grounded")