The Thieves of Ostia by Caroline Lawrence
 
Flavia Gemina is a natural at solving mysteries. The daughter of a ship's captain living in Ostia, the port of Rome, in AD79, she and her three friends, Jonathan, a Jewish boy (and secretly a Christian); Nubia, an African slave girl; and Lupus, a mute beggar boy, must work together to discover who is beheading the watchdogs that guard people's homes, and why.