The Ring that Caesar Wore by Ashley Gardner

Leonidas is coerced into a dice game at his local popina, with a stranger who turns out to not be such a stranger after all. His presence puts all around him in danger, and Leonidas struggles to keep the man’s identity secret.
Leonidas has no wish to be caught up in this trouble–he’s about to begin working as a builder, his ticket out of life as a gladiator and bodyguard.
The next morning at the building site, Leonidas almost immediately stumbles across a valuable ring buried in the mud, a ring that has the potential to change the destiny of Rome. Nero commands Leonidas and Cassia to uncover everything about this ring, including who owned it and how it had come to be where it was found. They will not be allowed to fail.
The ring proves to be only the tip of the troubles Leonidas uncovers, with danger from the Emporium to the Esquiline and some that visits Leonidas and Cassia very close to home.