By far the hardest interactive fiction I have ever played. It is just a series of puzzles, represented by characters. Puzzles include Euclidean geometry, wordplay (similar to cryptic crosswords), and a maze.
Some of the puzzles are bewilderingly difficult (such as the name of the Mark of Water). I don't think that it was ever intended to be solved without cheating. Using a decompiler gave a few answers that I could not otherwise get.
This is a pure mental-exercise game with no plot. I've brought up some of the puzzles on Stackexchange and Reddit, and it provoked some good discussion, so this is a good game to mine for interesting puzzles.
Review for Steam Edition:
Anchorhead is a masterpiece of interactive fiction. In this well-illustrated Lovecraftian game, you have to piece together the history of your husband's family as you move to a new town with a dark history.
This edition fixes a lot of the worst puzzles from the first edition, especially the very difficult mill section. It adds some new puzzles, too, some of which I found quite difficult (such as the dinghy), and others less so (the new opening sequence).
The illustrations are very well done, and go a long way to making this worth the purchase price. I love this game, and I'm glad to see it in such good form. I also appreciated the change in the orderly's magazine, which made me laugh. Some of the older texts in the game contain echoes of Lovecraft's racism, and they seem to be written new for the game, not old texts quoted, so I thought I'd mention that.
Earlier Review:
Anchorhead can completely draw you into its world. The writing and atmosphere are classic Lovecraftian horror, beginning as merely dismal and developing slowly into madness. Early scenes take on far different meanings on a second playthrough.
That said, this is a very hard game. I'm not sure how anyone could solve the (Spoiler - click to show)telescope lens puzzle on their own.
However, the depth of the game and the quality of the writing is such that it is still enjoyable even if you have to resort to hints from time to time. Many of the best moments are also the easiest puzzles.