Spirit Gates

The Spirit Gates were seven gates that were hidden deep below Duel Academy and were used to protect the Sacred Beasts. They could only be opened by obtaining the Seven Spirit Keys, which were only won in a Duel. Nearing the end of the first year of Duel Academy, the Spirit Gates were opened, and Kagemaru announced that the gates could be opened by possessing enough Dueling Spirit. He said at that point that both the Spirit Keys and the Spirit Gates were a ruse in order to provoke Duelists' to Duel each other over them and generate the spirit that was necessary in order to open the Spirit Gates. Yubel was later able to break the Spirit Gates in Season 3 and claimed the Sacred Beasts.