[jwhitlark]

David Drake wrote a sort of prequel to "Lest Darkness Fall" called "To Bring the Light", with an imperial Roman woman transported back to the founding of Rome. Drake's personal experiences increase the realism of his writing, IMO.

He also wrote "Ranks of Bronze" which is sort of the reverse idea, a Roman Legion kidnapped by aliens for preindustrial warfare. Absolutely worth a read, and an interesting counterpoint to most of the 'bringing civilization to the barbarians' narrative these stories normally follow.

H. Beam Piper did a bunch too, check out "Lord Kalvan of Otherwhen" if you can find it.