Secret Dungeon
In Ultima: The Black Gate (SNES) there is a secret dungeon that is accessible from The Dungeon of Unknown Fears. The secret dungeon contains the Shield of Reflection and also has connections to some of the other dungeons in the game - Dungeon Sagatious, Undertaker and Buccaneer's Cave
Secret Dungeon Notes:
Not sure what any of the switches do individually. But generally switching them will open the unexplodable walls marked with green.
There are more switch rooms and many many more switches than are necessary to open all the unexplodable walls. Some of the switch rooms also have tile-sets and monsters that are different from the rest of the dungeon. Is it possible that some of the switches have effects outside of the Secret Dungeon? I believe that there are some doors elsewhere in the game that appear to be unopenable. Do some of the switches control them?
The golden key allows you to get back into the secret dungeon if you use the exit to Undertaker. There is also a golden key in Undertaker that will let you get into the Secret Dungeon.
If you enter Undertaker from the Secret Dungeon you don't need the Caddellite Helmet. But, if you leave Undertaker through it's normal exit you will need the helmet to go back in.
If you exit to Buccaneer's Cave or Dungeon Sagatious you can't get back. (At least as far as I'm aware)
The Boss Key doesn't appear to have any use in the Secret Dungeon. I assume it's so you can exit the connecting dungeons more easily or to stop you getting stuck in them.