I only watched this picture because of my current obsession with Charles Dance. It's a very strange film, rather too artsy and noir for my taste, but in general a pretty good picture.
A few items: far too much is made of the "hidden city" component. Any older city has subterranean tunnels and chambers. Disused subways, dry drains, utility shafts and connectors, power cable channels, catacombs, and sewage pipes form a veritable honeycomb beneath a city's surface. Not secret so much as not relevant to the daily life of ordinary people. That these areas might be useful for file storage is sensible and efficient.
The more intriguing mystery for me was why the writers writers (who otherwise did an excellent job) failed to give us explanations to these questions:
Why would an intelligent, sophisticated, educated man disregard several days' worth of scheduled meetings in order to assist a complete stranger in an ill-defined quest for information of questionable value?
Why, having learned the mysterious government secret, do the pair do absolutely nothing with what they've learned? It's implausible that anybody would go to so much trouble merely to satisfy idle curiosity.
And finally, why do the pair decide to continue rummaging for secrets?
On the plus side, "Hidden City" does include a very excellent performance by Charles Dance. I think that one of the more difficult challenges for an actor is speaking lines that convey information the audience needs but would never be uttered in real-life conversation. The film contains an astonishingly large number of such lines, and Dance delivered them so naturally that only when I'd finished watching did I realize. In fact, I can't think of a better example of that sort of acting. Well done, Charles Dance!