Bottom Line Up Front. This is not a FATE Core friendly game. Despite a text box in the book stating that it is. This game is firmly entrenched in the system used in Diaspora (that came out before FATE Core).
I really wanted to give this product 5 stars but I can't. I love the Arthur story. From "Le Morte d'Arthur" to "Dragon Lord" the various incarnations of Arthur has held my imagination. My hat is off to the authors. There is love in this book. There is thought and skull sweat present in spades...however, it is not current. It will require users of FATE Core to put in some serious thought and work to make this system compatible.
There is no Success with Spin. You pay your Fate Points for Aspects prior to rolling not after. You don't have Free Invokes or Boosts. Create Advantage doesn't exist. It is more SOTC-like; using maneuvers. Characters start with 5 free stunts and 5 refresh (reminiscent of SOTC). You can purchase more refresh with your free stunts. Stunts don't work quite the way they do in FATE core (you have to power your stunts with Fate Points) so you have to have the extra refresh or you won't be able to use your stunts (there are a few generic +1 stunts called specialisms, but these are few on the ground). You can still have aspects that are compelled to earn Fate Points, but since you have to power practically all your stunts with Fate Points you need a big supply straight out of the box. The mass battle system is intriguing and reminds me a lot of the Savage Worlds mass combat system. It does not match any version available from FATE Core (such as in the System Tool Kit book) Any FATE Core vets are going to have to do some serious work to adjust the engine to work they way they are used to. Warbands and battles get special attention, but since you aren't using FATE Core teamwork bonuses aren't used and number crunching becomes more prevalent.
Having said all that the setting itself is intriguing. Dark Ages Britain is a completely "open" time period. There are no real accurate historical records from the time the Romans left until two or three centuries later. (Bede or the Anglo Saxon Chronicle) These came out in the early to late 700s almost 300 years after the Romans left Great Britain. So little can be said for the historical accuracy of the setting other than there isn't any. That isn't a bad thing. It allows myth and conjecture to inform a compelling story about Player Characters facing off against invasions from the Saxons and from Hibernian pirates, while keeping an eye out being stabbed in the back by other would be British High Kings. Just about every aspect of the game is some sort of cultural or religious clash. Romano-Britains against the Saxons (or Hibernians) Christians against Pagans (or other types of Christians).
If this book were to get a re-write I would suggest getting rid of the dated engine. Then plug in the FATE Core engine. Get a bit more art. Change up the blocks of text to columns to de-stress your readership. (It was hard to read more that a page or two at a time). Give more examples. Of everything. Most people are not experts on the period. Move the setting to an earlier part of the book, or at least add some fiction so that the readership knows what the game is going to be about before they start making a character. This change is in fact critical if you expect Players to use the FATE Core method of setting creation prior to character creation.
The price point is too high for a game book that uses an older version of FATE that the majority of readers will find necessary to change. Any changes will necessarily be extensive and a 15 USD price tag will punish users of the FATE Core system who want to use this setting. In its present state this book may be worth 5 USD and certainly not more than 10 USD.
The setting sells the book. The engine detracts from it. Therefore I'll give this book three stars.
Edit: There is a rule for spin...in general if you get spin you get to add +1 to your roll. Again this is not typical of FATE Core. There are quite a few more dissimilarities but I won't burden the reader any further.
[3 of 5 Stars!]