"When Redwall Abbey is attacked by a rat known as Cluny the Scourge, the Redwallers rise up in defense. A young mouse known as Matthias embarks on a quest to recover the legendary sword of Martin the Warrior for Redwall and his friends, because he believes that the sword is necessary in defeating Cluny's horde."
The story didn't have to be about mice at all though, it could have just been a standard high fantasy epic.
Really, the animals basically serve the same purpose as fantasy races throughout the series. Like, all hares have similar personalities, all badgerlords are essentially alike, etc.