The outcast blade - Jon Courtenay Grimwood

As the Byzantine and German emperors plot war against each other, Venice's future rests in the hands of three unwilling people:

The newly knighted Sir Tycho. An ex-slave and trained assassin who defeated the Mamluk navy but cannot make the woman he loves love him back. Tortured by secrets, afraid of the daylight, he sees no reason to save a city he hates.

The grieving Lady Giulietta. Impossibly rich, deeply spoilt. A virgin, a mother, a widow . . . Both emperors want her hand for their sons in marriage. All she wants is to retire from the poisonous world of the Venetian court to mourn her husband in peace.

And finally a naked, mud-strewn girl who crawls from a pauper's grave on an island in the Venetian lagoon and begins by killing the men who buried her.