It was a secret hiding place located in the wastelands and protected by, along with a reality lock, a one-second time shift into the future. The Foundry made use of dimensional technology, making it larger on the inside than externally. Within its chamber, Rassilon tampered with Gallifreyan physiology in order to give them life after death, a secret that at the time only the Vampires possessed.
The Foundry contained many technological wonders that were later used by the Time Lords, from a statue of Nemesis to the De-mat Gun. There were even star systems trapped within the structure, home to civilisations that Rassilon felt were a threat to him; one of these was the Divergence.