Castle of Dr. Brain is the first in a two-part series of educational puzzle-games from Sierra. This brain-blasting game is set, how surprisingly, in Dr. Brain's castle, where the player will have to beat Dr. Brain's traps and puzzles in order to fulfill the assignment given.
In the game, there is a rubber tree in the third floor hallway, and if you examine it, the second sentence of the message will read as follows: "These can be very useful should you fall off a high cliff." This must be a reference to the Sierra-style "death" in The Secret of Monkey Island.
If you look in the mirror (in the corridor right behind the castle entrance), the comment says: "How does that go? 'Mirror, mirror on the wall...' No, that only works in Mixed-Up Fairy Tales.