Tsarina is a long read, with complicated Russian names and geography that may not be familiar to everyone (I required some research.) The story is very intense for someone unaccustomed to the violence, war, and rape often found in historical fiction books. The first half of the book contains graphic depictions of rape (to which I almost DNF) whilst the second half of the book involves horrific descriptions of torture and death. Only read this book if you are prepared to face those challenging scenes - they are pretty inescapable in this one. 
That being said, I loved Ellen Alpsten's writing style. She clearly has a passion for this period in history and all the characters she discovered there. The book was evidently well researched, with many quotes now underlined in my copy. I have learnt a lot about Russian history, customs, and cities which I had previously been ignorant of. Catherine (Martha) is a well developed main character, and the discussions of femininity, fertility, and family in the midst of war were emotional to read. 
I would recommend this, but go into it prepared for a battle through hardship (as Catherine always was.)