This is a really expansive adventure! Some of the puzzles were real head-scratchers for me; I played on and off for a few days before I finally reached the end, and even then I needed some hints that I found in this thread. I think that there were only two places that I felt were a bit unfair.
First was in the water-filled passage in the bottom level. I could only enter it by SWIM PASSAGE, or GO PASSAGE, but not SWIM WEST even though to get out you need to SWIM EAST or SWIM WEST. And just plain SWIM didn't work, even though that will happily get you drowned in a couple of places earlier in the game.
Second was entering the pothole, again in the bottom level. Neither GO POTHOLE, ENTER POTHOLE, nor SW worked, so I tried for quite a while to enlarge the hole. It was only when I read Garry's comment above that I discovered how to get into the crevice.
Other than those two small points, this felt like a pretty difficult but fair game to me.