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 the game is most likely an artificial one, created as a means to enter into Kingdom Hearts and as a source of immense darkness and power. Xehanort eventually succumbed to darkness and Maleficent took over the castle with Heartless nine years before the events of Kingdom Hearts.