Did you happen to press the arrow keys quickly while moving through the dungeon? Due to the nature of the game engine, spaming the key may sometimes cause certain events or monster encounters to be unintentionally skipped. You need to defeat four monsters to collect four keys. If an event or encounter seems to have been skipped, please try using the “go back” button to re-enter the area. If you have a save file from when you first entered the 9th floor, I recommend loading that and proceeding more slowly to trigger the necessary encounters. Otherwise, using the back button may help reveal if anything was missed. I apologize for the inconvenience!