Other reviewers have already gone into detail about the novel controls, so I will pass on that part. However, even without the mechanical inventiveness, this would still be worth playing.
The world of Amaranth has a unique, dreamy ambience. Despite its small size, there is enough detail (for example, the books) that it doesn't feel sparse. The responses to entering the (unnecessary) compass directions are an excellent touch.
The writing is very good - it falls into slight cliché once or twice, but it's clear and often very vivid. While I agree with other reviewers that the Red Prince does come off as a bit too passive, he is a memorable antagonist.
While the horror element is unlikely to keep you awake at night, the game does a good job at being eerie while staying away from cheap tricks: there is very little violence, little squickiness (apart from the undead guards), no unfair deaths. I enjoyed some of the dreamlike and unnerving images, such as (Spoiler - click to show)the horned skeleton at the shrine in the forest.
However, as the above paragraph shows, one could argue that the protagonist is a bit too safe in what should be a game about battling a powerful nemesis. The downside to the unique control system is a sense of alienation: we view this world top-down, like an intensely detailed model village. The PC can go pretty much anywhere: movement is carried out by the verb EXAMINE. At first, after the intro text's mention of dreams, I assumed that the game was meant to take place inside the PC's dreams, explaining the alien ambience that stems from the controls, but since you can go to sleep and dream in Amaranth (activating a clever, well-written hint system), that doesn't seem to be the case.
The puzzles are mostly simple and well-worn, in contrast with the innovations in other aspects. This isn't a problem for me (I'd rather play a game with conventional-but-logical puzzles than one that forces in the puzzles), but neither is it an advantage. However, one puzzle I did find original was (Spoiler - click to show)how you dispose of the Red Prince's body.
Castle of the Red Prince is a delicate, gem-like petit-four rather than a full meal. Highly recommended if you feel like playing a Gothic fantasy game that is a small time investment and unnerving rather than gruesome, as well as mechanically innovative.