The Horn - the Engineers are the stars of this colony's future
The Six Families made a deal with the Anvarrid invaders. In return for leaving their Fortresses untouched the psychics of the Families would become warriors to guard the Anvarrid. They would share their technology and give up their individual languages. The Families lied and rather than share technology that could be misused they hid it. Also, one promising engineer in each generation was designated the Oathbreaker and taught the language of the Fortress so they could communicate with it. Then the Horn Family discovers a man freezing to death on a glacier within sight of the abandoned Salonen Fortress. A man with no memory, probably a spy sent by a hostile empire, of pure Salonen DNA, and with an instinctive knowledge of engineering. Killing him and burying Salonen would be the safe choice. Or the Horn might risk their independence, the Treaty, and their way of life to recall the technology of the founders and bring Salonen back to life.
Amal, Lady Horn, is the Anvarrid governor of Horn Province but prefers her role as engineer and Oathbreaker. Twice a year she and her year-mates risk the dangers of the glacier to ensure the defunct Salonen Fortress remains hidden and this time they find a man nearly frozen to death in his camp. To Amal it seems obvious that the Cince sent him to find a way into Salonen and steal any functioning tech but sending an amnesiac makes no sense. Horn Fortress tells her that the man, Dalyan, is of the Salonen Family and that makes sense because a Fortress will only open for its own Family. The Cince treated him more like a prisoner than the Horn and they've even been sending assassins after him every month like clockwork. Even so Dalyan fears Amal will kill him if she discovers his true origins. The Cince lied when they told him he lost his memory due to illness. He snuck around their compound and saw that they grew people in a machine. But as the Oathbreaker Amal had suspected that from his DNA and knows it makes him the possible salvation of not only Salonen but also the other slowly failing Fortresses.
The Horn trilogy reaches back to the original colonization of their planet. Should the Horn maintain the current status quo or reclaim their forgotten science and technology. Some of that knowledge is potentially dangerous. The Infirmarians are appalled to learn that the Oathbreakers knew the Fortresses could manufacture new cures for diseases but one Family experimented and was wiped out by a super virus of their own creation. The Cince dabble in morally questionable genetic engineering but the original colonists were in fact all genetically altered humans because it was necessary for survival on a hostile planet. The logistics of the colonization are unique and interwoven with one man's determination to right a personal tragedy.
The Horn trilogy consists of Oathbreaker, Original, and Overseer. It takes place in the same world and at the same time as the Palace of Dreams series. The Horn Trilogy is self contained but it is referenced in the Palace of Dreams series although its not required to understand them. Its available in individual eBooks and paperback as well as an omnibus edition.