Disclaimer - I have known the author for over twenty years, but will do my best to make this review fair
"The Doorway in the Hill" is, as the description says, more than just a supplement. Taken together with the album and visual accompaniment that can be found on Mournful Leper's website make for an experience; an experience that is well worth the price. I won't go too much more into my thoughts about the album or the video here, as I'd hate to put anything into anyone's mind before they get the chance themselves.
I would like to talk about my thoughts on the supplement. It took me a little while to feel comfortable writing this review, as it took me a bit of time to get a feel for the "For Gold & Glory" (FG&G) system. The feel of FG&G is complete throwback, to when I was playing the TSR Goldbox series and just getting into what TTRPGs could be. Pouring over tomes and figuring out tables with advanced math as a pre-teen is good memories, and FG&G gives me those feeling. But that investment can be quite a bit, as even while the system hearkens to the old school, that style of TTRPG is dense, and asks you to mine it for what you're after. Your enjoyment of FG&G will color how much mileage you will get out of this product. As to "The Door in the Hill" itself, the setting is described in just enough detail that a GM can fill in the gaps with whatever it is that they want to throw at their players. The hooks are there, together with the race and class additions that meld well into the region and its story. I enjoyed lore that left you in a region that has forgotten, or tried to forget some horrible things in its history, and has many equally nasty things looming on the cusp of tomorrow.
The writing is good, clear in the references to the source book. If you want to try your hand at weaving yourself through vines that thirst for blood, and enjoy the FG&G system, I'd happily recommend giving this a try.