Mercer Mayer's illustrations were part of my youth. They really brought the Great Brain series alive--which itself could make some good text adventures, with Tom D.'s scheming and puzzle solving. So when I confirmed he was the author of this text adventure nobody'd written a solution for, I figured it'd be fun to try.
Angelsoft parsers, though, tended to be not quite up to Infocom's--undoing doesn't work, and the randomized responses for nonworking verbs are just baffling. And they didn't have those neat InvisiClues. Which were almost necessary for Forbidden Castle. Mayer imagined a very cute world: a gnome with a weird belt, an ogre, a fairy, and other things that'd been done before, but the real charm of this game is how you can ride a dragon or pegasus to places you need to get. The whole map is connected at the end, but you don't see how at first--and there are plenty of weird deaths in the isolated areas if you do things wrong.
And you need to learn what magic items do--you're not told. Some help you around some magical beasts but hurt you around other. Wearing a sword gets you killed. Insta-deaths zap you a lot here, and it's not clear why. You can also assume it's a good idea to (Spoiler - click to show)pick up the bag on the first move, but if you don't, you're totally lost. The game gets in trouble a lot here.
The technical annoyances and Player Bill of Rights violationscan't quite obscure the imagination, though, so a walkthrough is recommended. But there is too much verb guessing and cheap death for an honest play-through. Most people won't have the patience for that.