Sorcerer's Edge. The Tethered Citadel Book 3

After all they've suffered, rebel sorcerer Raythe Vyre and his fortune-seekers are still empty-handed, but they've found real treasure: peace. Deep inside the Ice wastes, Raythe's people stumbled upon Rath Argentium, the legendary Aldar city, and the long-lost Tangato people. After fighting through betrayal, treachery and powerful magic, they forged a hard-won treaty with the Tangato and their extraordinary queen, Shiazar.
Now they've put aside their dreams of wealth and revenge and embraced something better: a life outside the tyrannical Bolgravian Empire. But the Bolgravian Empire never gives up. The empire hasn't forgotten Raythe Vyre, and his enemies know where he is. Guided by Toran Zorne, the implacable imperial assassin, they are coming to claim Rath Argentium for themselves. Raythe and Shiazar know all too well that courage and cunning won't be enough this time: they are outnumbered, out-gunned and out of time.
Faced with total annihilation, it's up to Raythe to find an edge .