Totally agree (well maybe I would be more lenient about the rating). I also played Calendra's Cistern 20 years ago and replayed it again. The City is massive and quite nicely designed. Just like you, I was surprised with a linear walkthrough requiring you to unlock further parts of map. This approach feels quite strange now, especially after I played FMs from Skacky, Nicked or Korthak. But again, their stuff is made a decade later.