I have used no mods.
I got the key, reported back to queen and bombed spectral slime. I obtained portal room keys and continued game as normal.
I did not need to enter the trapdoor north of the trapdoor with spectral slime. Presumably this is mean to take me to Insanitarium / Archives Dungeon or a lower level to then get to it.
Upon interacting with it, it says, 'I lack the keys for the lower dungeon'. I do have Lower Dungeons Key. I need the scroll of rapture for the plan in part IV which I need to go to Insanitarium / Archives Dungeon. Perhaps the text for lower dungeon key is wrong, and I need a key for Insanitarium / Archives Dungeon which is not specified in the walkthrough I found?
The walkthroughs I found were version 0.9.1
Viewing post in The Princess And The Tower (18+) comments
Thanks for the report.
I checked the code. The message you encountered can happen if you lack the keys ITMKeyDungeon2, ITMKeyDungeon3
["ITMKeyDungeon2", "Lower Dungeons Key", "The", "key", "A rusted key, which provides access to the deeper dungeons.", None, 500, ["no sell", "unique", "no find"],{}],
["ITMKeyDungeon3", "The Pit Key", "The", "key", "A rusted key, which provides access to the deepest reaches of the dungeons.", None, 1000, ["no sell", "unique", "no find"],{}],
ITMKeyDungeon3 is an endgame item for the Goblin King ending, so it might be that one? That can be opbtained from Gwynn if the Goblin King Quest is unlocked.