The Amazon editorial review implies this film is all style no substance, but that is untrue. Although a lot of effort went into the style (partly in homage to Fritz Lang's 'Metropolis') the darkness here is no mere plot device, it is a metaphor.
The 'dark city' is an elaborate device created by a soulless race of aliens (who all look like John Gielgud, perhaps because they "use our dead as vessels") as part of a giant experiment to find out what makes humans IRREDUCIBLE (i.e. the part of us that can't be measured or quantified - our souls). 'Dark City' is clearly influenced by Gnostic spirituality, with its concept of a demiurge who has trapped us in a materialistic 'circle of circles'.
The film is tightly scripted and packed together, with extremely fast camera shots (like British show 'The Prisoner', with which it has much in common), and more than one viewing may be necessary to understand the plot in its entirety.