The plot points I remember are:
At the start, there are two armies about to fight, one has eagles(the good guys) and has never faced the cannon of the opposition. The battle goes badly for the good guys as their eagles are shot down by the cannon/some sort of ranged weapon.
Later in the book a character discovers prisoners from the battle being used as slaves.
I think is might be similar to the Redwall books, in that the characters are anthropomorphic animals. Much kudos to the person who can identify it from those vague details!