Fun Fact: Shadowgate was another NES game adapted into a novel for Nintendo's Worlds of Power book series.
Shadowgate is a port of a point-and-click adventure game that was originally designed for the Apple Macintosh computer. If you can imagine Zork with graphics you have the idea. Much of the game took place through text-based storytelling. Interaction with the environment was presented both as a series of text commands (Look, Use, Hit, Open, etc.) and the ability to click on objects through the point-of-view window on the screen.
The game played out as a series of puzzles encountered while the player explored a castle and tried to find their way to the Warlock Lord. Shadowgate was extremely unforgiving to the point where the NES port, ostensibly in an attempt to make the game more friendly to the console audience, included a hint system. Unfortunately the further one got into the game the less useful the help system was, and failing to solving puzzles correctly got you killed pretty much every time towards the end.
For gamers who didn't own a home computer, Shadowgate may have been one of their first exposures to adventure games, which helps explain the lasting popularity of this NES title.