(This review is a slightly condensed version of the one that appeared on my blog during the IF Comp 2009.)
The game is written entirely in a mock-archaic style, which might get old(e) very fast if you don't blank it out while reading. It does contribute to the silly atmosphere, but the author might still have achieved the intended amusing effect, if he had restrained himself and had used the style only in NPC dialogue and in the integrated hints.
Depending on one's tolerance of (silly) puns, the writing might be found groan-inducing or hilarious. It's not highbrow literature, but that's not what it aimed at, and I felt it was filled with a sense of good-natured humour.
The story/plot is thinner than a Chinese lantern, but that doesn't matter too much, because the game does not pretend otherwise. Still, I'm somewhat on the fence: On the one hand, it would be nice if there was a bit more world-building or backstory. On the other hand, that would probably run counter to the game's overall style, which is a deliberate mix of traditional gaming flavour and self-aware silliness. Adding more backstory might counteract the game's lampooning of the treasure hunt genre and of IF conventions. The (Spoiler - click to show)"intricate object", for example, is clearly a parody of how some other IF games force the player to examine everything in several levels of detail. Giving that thing a credible purpose within the story-world would obviously subvert the parody, and the same goes for the motley collection of treasures.
The puzzles are mostly straightforward, sometimes allowing multiple objects for the solution. Some reminded me of the LucasArts adventures of yore.
One really cool feature is the ability to retreat from the castle at any time with the >WIN command, whereupon you'll be presented with a short account of the rest of your life. Your fate ranges from (Spoiler - click to show)an early death by stamp-poisoning to a nobleman's life, and there's a different result for each number of collected treasures.
Astounding Castle is quite long, which is good, but its later stages suffer from lengthy walks to and fro. There's also a maze, which didn't add much except a pinch of old-school flavour.
I find it hard to rate this game accurately. If four stars are taken to mean "A very good game that's almost an IF milestone", then Astounding Castle doesn't qualify. On the other hand, I really had fun with it, and three stars make it look too mediocre, so I'd rather err on the side of generosity.
Summary: An enjoyable, merry game if you're in the right mood to get into the silliness.