Salt & Sacrifice by John Carlyle O'Neill

Exiled for their beliefs, the faithful have been imprisoned deep inside the salt mines of ancient Rome. For the empire, salt is a symbol of life and prosperity; for the Christian slaves who mine it, its only meaning is death. Barbaric persecution and cave-ins threaten to stamp them out entirely, but beyond the shafts their outlawed religion is growing more quickly each day. Against the backdrop of rebellion and rising religious tension, Pallas, a retired master builder and former soldier, is searching for a mysterious engineering prodigy. His quest delivers him into the merciless jaws of the violent and corrupt salt trade, the cornerstone of Roman political power. A determined traitor has acquired the means to exploit a war-torn province. This vicious tyrant is poised to strike, and Pallas must find the strength to stand against him.