Emperor: Time's Tapestry #1
By Stephen Baxter

It begins when a Celtic noble betrays his people at the behest of his mother's belief in The Prophecy and sides with the conquering Roman legions. For the next 400 years, Britannia thrives-as does the family that contributed to Rome's reign over the island with the construction of Emperor Hadrian's Wall and the protection of Emperor Constantine from a coup d'?tat.

And even when the sun begins to set on the Roman Empire, The Prophecy remains. For those capable of deciphering its signs and portents, the future of Earth is in their hands