The Demon Count by Stuart, Anne

Young, impetuous Charlotte Morrow was embarking on a new life in Venice, the City of Lovers. Newly orphaned, yet filled with more excitement than grief, she had barely arrived in the cosmopolitan city when Holger von Wolfram, the dashing Austrian captain, Jean Baptiste, the French aristocrat, and Mark Ferland, the handsome British officer, made known their desires to possess her golden-haired beauty.

But at Edentide, her guardian's immense palazzo on the Grand Canal, Charlotte soon had reason to regret she had ever left England. She was a prisoner trapped in amaze of mysterious corridors and dark, mirrorless rooms. Between sunset and sunrise, in a nightmare world of evil, she was helplessly drawn to the passion of the Prince of Darkness - the Demon Count