Final Keyhole

The Final Keyhole is the last Keyhole seen in Kingdom Hearts. It is in Hollow Bastion, where the Princesses of Heart are in their dormant states, and all of their hearts are required to open the door to Kingdom Hearts. The Keyhole is not truly unlocked until Kairi's heart is restored by Sora after his second fight with Riku. Sora had given up his own heart to save Kairi's and the other princesses'. It was revealed by Maleficent when she said "The princesses of heart are all here. It must be her," that Kairi was also another key to opening the heart-shaped Keyhole.

When this Keyhole was revealed, several powerful Heartless were released. These were the Clock Tower Phantom, Kurt Zisa and the weakest of them, the Behemoths. This Keyhole is only sealed during Sora's second visit to Hollow Bastion. It is also said that the Keyhole possesses unlimited power and wisdom, which explains why Maleficent wants to collect the princesses so badly.

Based on the Ansem Reports, Xehanort had opened a door deep beneath the castle, allowing the Heartless to overrun it, destroying Hollow Bastion and breaking the barriers in between worlds. Thus, the Final Keyhole in this game is most probably an artificial one, created as a means to enter into Kingdom Hearts.