Greatest Legacy of the Magus Universe - Chapter 628 Council of Seven

... that Saratoga had created it through magic based on his memories.

Saratoga's remnant will gradually start to fade away into tiny motes of light. He threw a circular disk at the youth and explained, "This will establish a teleportation circle to this vault wherever you place it.

"But the gate cannot be too far from the Castle. Make sure to place the disk in a secure location. Your mana signature has already been imprinted onto the disk, meaning only you can enter this vault."