Does Dale fall into the palace dungeon?
Dale falls from the red room where all the evil spirits are, through space, to a weird platform at the bottom of the palace. Typically dungeons are at the bottom of castles and palaces. Perhaps the Fireman set up the platform to catch all the bad spirits that fall through space (Dale is not bad - the doppleganger tree pushed him). That makes sense why the glass box is set up to capture spirits that fall through that particular location - to intercept the bad spirits as they fall on their way to the dungeon (the box was set up by either Jeffries, Cole, Cooper, Briggs or somebody to try and catch Jao Dei).
The "mother" creature cannot open the door - it is locked, unopenable on both sides because only the Fireman has the key.
Perhaps the reason Naido flips the switch is to change the cell, so perhaps we are in "cell 15" with Naido at first, with the monster banging on the door. Then she flips it to "cell 3" , American Girl's cell, originally intending to get them away from the monster. But then she falls off unexpectedly. Why she has access to this lever I don't know. Perhaps she wasn't meant to switch it which is why she is zapped and falls of.
Later Naido transforms into a Diane, who is dressed red room style - suggesting she is an evil spirit who was held prisoner in the Fireman's dungeon.