This is a quite amazing story and is a great read, although my view is that it is a bit long. I would have preferred it to be compressed into about 400 pages.
The concept of bringing combatants and brains from various eras of Earth’s history to save the planet Arden, on the opposite side of the galaxy, from the invasion of the “Horde.” The horde appear to be an almost limitless force of suicidal beings, who are driven by telepathy and have psyche which is completely contrary to anything human.
The convergence of the best of military strategies and tactics, even from factions who were sworn enemies back in the Earthly eras is very interesting. The simplest and most ancient strategies, tactics and tools often prove to be better than modern or futuristic methods.
For me, the story really picked up its pace as soon as we gained visibility of the minds of the Horde. From that point on, there was high tension from every viewpoint right up until the end. I can’t say much more than that without creating spoilers.
I recommend this book to all lovers of fantasy and sci-fi, although you will need some stamina to get through it.