A mighty fortress - David Weber

Young Cayleb Ahrmahk has accomplished things few people could even dream of. Having smashed a hostile alliance of no less than five princedoms and won the hand of the beautiful young Queen Sharleyan of Chisholm, together they have created the Charisian Empire, the greatest naval power in the history of Safehold. Charis is transformed into a place of refuge for all who treasure freedom. But their success may prove short-lived. The Group of Four, the clique that controls the Church of God Awaiting - and, through the church, most of Safehold - has decreed their destruction. Mother Church's entire purpose is to prevent all to which Charis is committed, and she will not rest until it has been erased forever. But there are still matters about which the Church knows nothing, including Cayleb and Sharleyan's adviser, friend, and guardian, the mystic warrior-monk named Merlin Athrawes. Merlin knows all about catastrophic battles against impossible odds, because he is in fact the cybernetic avatar of a young woman named Nimue Alban, who died a thousand years ago. Mother Church's juggernaut rumbles down on Charis, but Merlin Athrawes and a handful of extraordinary human beings stand in its path. The Group of Four is about to discover just how potent the power of human freedom truly is.