The tower is the Tower of London. The zoo is a return of the Royal Menagerie to the Tower after 165 years. And the tortoise is Mrs. Cook, the world's oldest tortoise, owned by Beefeater Balthazar Jones who resides in the Tower with his wife and the memory of his young son.
Balthazar Jones and his wife, who works at the London Underground Lost & Found, are still struggling with the sudden loss of their only child, when Jones is assigned to manage the new Royal Menagerie at the Tower. Jones turns to organizing the Menagerie to deal with his grief, while his wife deals with returning items to their rightful owners to keep herself from dwelling on their loss. As they grow estranged, we learn more about how they met and fell in love.
Although there is sadness in the book between Balthazar Jones and his wife, Stuart tries to keep it lighter with the antics and assignations of the other characters. From the enraged Ravenmaster who is disturbed that other creatures should outshine his ravens, to the Minister of the Tower's Chapel and his unrequited love for the barmaid, there is a lot of fun in the book. Stuart never lets the sadness become overwhelming.
Even if you have been on one of the Beefeater's tours of the Tower of London you will learn more about the history of the place and how the Beefeaters (or, as they refer to themselves, Yeoman Warders) live within the confines of the Tower. A light, delightful read even if you have never visited the Tower of London.