I am not familiar with the initial writing of Carmilla so I will be reviewing this as a work in itself, which applies to the narrative as well.
Not many mediums have what it takes to impress me positively from the very beginning, but you managed to grasp me from before the story was even to begin. The introduction, pertaining to a remarkably competent allusion to spiritual science, is something I should not expect to find in such a work, even if this is technically not a game at all, and it did make me very keen to read on further. The writing itself is quite great and expressive and it does not seem like it was merely stolen from a book. There are a couple of mistakes scattered throughout but it is manageable. The one Shakespeare reference was very well received.
The visuals are naturally excellent, though the moving vistas sometimes do not overlap properly giving it an unnatural staggering. The music well complements the scenes, yet is slightly repetitive, specifically since I took my time reading. The final track with a vocal presence was not really fit for the conclusion, it should have gone somewhere else. The mechanic of the notebook entries is welcome conceptually, but it did not have nearly enough of them to fully justify it.
Speaking of which, due to the nature in which the story was building up mystery and expectation, the conclusion struck me as rushed. Before I began the final chapter I was to believe that a more dramatic twist will affect the story, but definitely not an end. I was reading the story with care and yet I can not answer to myself certain puzzling elements, such as who the lady transporting Carmilla was and why she specifically claimed to know the General, as well as whether she will return or not. Carmilla's now perplexingly false affection towards Laura strikes me as shallow since her character was not developed enough throughout the story to amount for that, and either saying that is was a charade or a conflicting desire is unsatisfactory . In fact, this was the case with all of the major characters, and through my reading I was specifically envisioning an upcoming change in Laura's attitude towards Carmilla's secret nature, yet which never occurred, making me reconsider the romance as superfluous. There are other letdowns as well, such as the mention of the Schloss' secret passages, certainly a hefty idea to just throw out, but which never actually amounts to anything. Also I did feel like, as I was reading, it was demanded of me to actively partake in solving this underlying mystery, which then proved to be of a mostly factual kind. As the General was unveiling his story the association with Carmilla seemed so forced that the answer surely must have been more expansive...; This is partly caused by the inherent dynamicism of visual novels but it could have been addressed in the writing.
Another thing to point out is that the spiritual aspects mentioned in the introduction and through the notebook entries ultimately crumble into nothing after the no longer doubted presence of vampires, a purely material manifestation. Yet this opens up the path of their conceptual articulation towards a mythopoeia all your own. So, as far as I am concerned, you will be doing this work a favor by further expanding to the present story, but since this was at its kernel a rerepresentation meant to point towards its more fleshed out narrative predecessor, the least I can ask for is a continuation in style and a retainment of vision.
In essence, I am quite happy with the result and I encourage you to continue making visual novels. You may in fact want to consider writing one that is purely your own. Regardless I will keep an eye on further projects.