The Fanglith Series #2: Return to Fanglith by John Dalmas

The Federation: Oppressive, dictatorial, and merciless to all who oppose it, the Fedration has become an Empire – and imperial fleets are expanding its brutal dominion to the colony worlds. Those of the resistance movement not trapped in the first strike barely escape with their lives.
Larn and Dineen: Their deepest hope is to join the rebel forces. But the base is hidden well – and in the meantime the two must lie low. No one will look for them on a barbaric planet like Earth…
Arno of Courmeron: The Norman knight who captures them learns that plans and wealth can be wiped out by the unpredictable…and that blood enemies may become blood brothers.