Two on a Tower was the 11th Thomas Hardy's 14 novels that I have read. Hardy can be depended upon to paint a vivid picture of the characters' environment, and their relationships to it, but this time with a twist: One of the two characters being an astronomer, most of the environmental descriptions are of the heavens, and are wonderfully appropriate for the characters' actions and 'aspects'.
Hardy had a gift of creating characters who are fascinating in their personalities and actions, and together with the environmental descriptions, reading his novels is just one step away from watching a really good movie of the story.
Of all Hardy's varied characters, I felt the most sympathy for the two on the tower. Viviette has a great need for love and is selfless in giving it. Swithin, a somewhat naive and literate scientist, is at the same time a tender and faithful lover. Of all Hardy's stories, I hoped that this one would somehow have that "happy ending", and I suffered uncounted times for both characters.
I highly recommend this book for emotional involvement, though it may tear you apart to read it!
I would also recommend another of Hardy's lesser known novels The Woodlanders, which I understand was his own favorite story, and remains mine also.