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sea-level was obviously on the upper level
That is correct.
I didn't realize there were "Esplanade Cottages", nor a lower structure that was the Old Reading Room and part of the whole ensemble.
This is why Brighton Bike Tag is so good (one of the reasons)... you learn more about your town!
When I was doing the tag originally, I was just thinking of the Alice / Lewis Carroll angle:-
https://www.theargus.co.uk/news/10915806.brightons-very-own-wonderland/
The listing says that the tunnel emerges 'between Esplanade Cottages on the other side'. I can't see them in aerial imagery. Are they just a façade built into the northern embankment, some kind of folly like The Temple, or hidden under trees?Scratch that, I didn't realise that Esplanade Cottages is actually the name for the southern façade on the upper level.