Pretty fun and interesting concept! I built a very wonky castle that went to the edge of the screen, I'm also glad the game let you build sideways. I like the theremin-style music, and the change when the Great One shows up. Also like that he's silhouetted, fits more with the unfathomable theme.
I did manage to "merge" some blocks together somehow, and the tentacles are giving out too many top pieces I feel, I think having to exchange a few for decorations is cool, but I had to exchange 10 or so before getting back to some squares at one point.