In the previous book, the humans have just been defeated soundly and are in full retreat to the lands of the Roum. The Bantag are in hot pursuit. The humans are counting on the winter weather to halt the Bantag campaign. However, since Ha'ark is from a different (modern) world, he understands the importance of continueing the campaign once the enemy is on the run. He pursues the humans and begins the seige in the dead of winter.
Ha'arks lines of communication and supply are tied to a single rail line leading all the way back to the Great Sea. Several hundred miles. The port city is resupplied via sea from the land of the Chin. Ha'arks supplies take 10 days to reach the front lines from the Chin. Ha'ark has plenty of trains and ships ferrying supplies.
I tell all this because resupply is the only Bantag weakness. If the humans can disrupt the resupply, even for only a few days, the tide of the battle will turn. That's all I will tell about the battle.
If you're reading the series, this one has to be read in the correct order. If you are not reading the series, a lot of the storyline may be confusing. There are lots of reference to events and people in the previous books.
Bottom line, this is a good book.