Caesar's Bicycle

From the back cover of the HarperPrism first edition: "Crux Ops to the Rescue! - The alien Closers are waging a war of annihilation and extermination on a million alternate Earths. To stop them, orbiting Hyper Athens recruits Crux Ops fighters from every human era. Private eye Mark Strang has battled the Closers in every century, with weapons old and new, from laser beam to crossbow. But now the Timelines are mysteriously closing, and he must storm the Roman Forum with knife in hand, risking the most hideous death of all - crucifixion!"