The Eternal Fortress (Star Legions: The Ten Thousand Book 6). Star Legions: The Ten Thousand, #6

The Eternal Fortress is a vast and impregnable bastion, built to protect the border. It is all that stands between the ships and soldiers of Black Legion, and their path to survival. Yet the Fortress is also a trap, designed to lure and pin an attacker, until superior weaponry can grind the enemy to dust over days of bloody battle. Xenophon has proven his worth, but he has few choices remaining. The long and bloody expedition has taken its toll on both the soldiers and the ships of the Legion.
He is aware that a retreat into the Empire will mean a confrontation with the Emperor. A confrontation which they cannot win. There remains only one alternative. To run the gauntlet of the Eternal Fortress. Already, the Medes are aware of Terrans proximity. They put a plan into action that will finally decide the fate of the Legion. It is a plan so deadly the Terrans will almost certainly die. Except those unlucky enough to be sold into slavery.
Or worse. The Eternal Fortress is the sixth book in the Star Legions series by Michael G. Thomas, the bestselling author of the Star Crusades series. Previously published as the Black Legion series.