I'm about a mile from this right now.
The grotto is unpleasant - absolutely, in my opinion, a Victorian (give or take) rich man's folly, per the Wiki.
The imagery is all faux-Mediaeval - mostly the standard utter nonsense Rosicrucian 'interpretation' of Kabbalistic symbols. It's very bullshit, edgy, Hot-Topic-Victorian silly stuff.
Except for where, at the Malkuth mark, IIRC, it opens up into a sort of temple-and-altar situation, where it is actually somewhat frightening, in the way that any secret Bacchanalian temple is, because nobody builds that sort of thing for a good reason.
