Seven Sisters Seven Sisters Trilogy, Book 1.

In an intact Ninth Century Roman Empire one man is given the chance to fight one more battle to bring an end to years of civil war. To win he must besiege the most highly fortified city in the empire. Fabled for its seven fortresses and ingenious water defenses it will be a very tough nut to crack. Aware that his devious emperor may sabotage him at any moment, he hasn't got enough troops or enough time but he must win for the good of the empire....