Tropes

Action Dad: Cadwallon against the Priests. Averted with Lleudd Ogrfan.
Adult Fear: Whenever a child is born. Slightly averted by the fact people were historically used to it, though just as saddened as today.
Aerith and Bob: Some Welsh names are familiar (Arthur, Gwen). Others are... not (Gwenwynwyn, Cadwallon, Gwrgwi, Maylgun).
After the End: Takes place in the direct aftermath of the collapse of the Roman Empire.
The Alcoholic: Caius after his recovery.
An Axe to Grind: Axes were the most common weapons in between spears and swords, bearing in mind the latter were still pretty common.
Anglo-Saxons: The primary antagonists, at least when the Britons aren't too busy fighting each other.
Ancestral Weapon: Swords were often passed down from family member to family member, given the wastefulness of burying it rather than reusing it. That said they would probably often be rehilted or have the blade fixed.
Attack Animal: In several parts of the novels (the Battle of Hadrian's Wall and the Battle of Mount Badon), dogs are used against the enemy by each side, respectively.
Audible Sharpness: Averted.
Authority Equals Asskicking: The Modus Operandi for both warrior cultures in the novel. An incapable leader who doesn't fight in the vanguard is quickly killed or ousted.
Avenging The Villain: Osla understandably wants to avenge his cousins Baldulf and Colgren.
Badass Boast: Played straight, given the concept of the arfayth.
Badass Family: Arthur's ancestors on both sides. To a certain extent, his children.
Bookends: Starts with an attempt to fatally wound Arthur and ends with a fatally wounded Arthur.
Brutal Honesty: The Saxons tend to be more direct and honest.
Cool Horse: Most of the Teulu retinues are equipped with these.
Cool Sword: Caledfulch.
Decapitation Presentation: It was customary to decapitate the head of a Pagan for his burial. It was also considered good to take your opponent's head as a trophy.
Deliberate Values Dissonance: Pretty much near constant.
Demythtification: Though a lot of the series is based on the historical texts more than the legends, there is some of this for tales like Culhwch and Olwen.
Duel to the Death: Happens several times.
The Dung Ages: Averted, as is historically accurate.
Enemy Mine: Several times.
Even Bad Men Love Their Mama: Cerdic, who loved his British Mother but hates his other relatives, like Natan.
Friendly Enemy: Sort of. Arthur is fascinated by the concept of a female warrior in the 6th century, thinking it an old thing of the past with people like Boudika. He is ready to pardon Aelswith but she orders him to kill her, to preserve her honour and her oath to Gwrgi and the Dog-Heads, and he does.
Friend to All Children: Ulfius. In his own gruff and taciturn way, Brastias.
The Fundamentalist: Several of them in all three encountered religions. Morgana counts as one (at least initially), as do people like the Bernicians' Angel of Death and Thunor, the crazy Saxon priest and husband of the former.
The Good King: Thoroughly averted.
Handicapped Badass: Several characters.
Here There Were Dragons: Both the Britons and Saxons believe in dragons.
Historical Domain: Most of the characters are at least semi-legendary, if not historical.
Historical Villain Upgrade: Sort of? We don't actually know if Cerdic was a bit of a madman, or if Aelle was a relatively nice and peaceful man (relatively! For both). 
Honour Before Reason: All the time. Honour mattered more than reason, though it didn't stop people breaking their oaths and fleeing the battlefield, or not bothering to avenge a family member or their fallen king (if oaths had been sworn to and by the latter). Though the effect if found out, that is likely becoming a pariah or even exile (and maybe even killed if serious enough!) was not particularly desirable.
Horny Vikings: Averted. The Danes who do appear in the novel are neither Vikings (they're actually mercenaries) nor wearing horned helmets, but appropriate Valsgarde helmets.
Human Sacrifice: It is mentioned that the Angel of Death practices this.
Hypocrite: Plenty of them.
Important Haircut: In British society, it was a sign of fidelity between clans and family members to cut one another's hair. 
Parental Substitute: Quite a few people were warded as children, as was customary to the Britons. This was to learn skills, learn to deal with others, make friends and strengthen ties between families/clans/kingdoms. It could also be useful for betrothals or marriages. 
Polar Opposite Twins: Eugein ap Urbgen Hen and his twin sister.
Post Humous Characters: Most of Uther's family. Gworthigern, who is the main cause of the whole mess.
Proud Warrior Race: Everyone. Everyone.
Redemption Equals Death: Often played straight. It was considered honourable to die for your lord/country. Especially in battle, and that includes for any reason.
Royals Who Actually Do Something: All leaders of the period and cultures in the novel were expected to lead from the front and fight personally in battle. Mortality rate for Lords and Kings was very high. Leaders only stopped *having* to be warlords in the 11th century. Reinforced by the fact that in the Migration Period, unlike say the 9th century, leaders had even less legislative power and were *only* warlords, like Arthur or Aelle.
The Bad Guy Wins: In some cases. Cerdic went on to become the current Queen's 36th great grandfather, and spawned the greatest English Kingdom, Wessex, and the one which united all of England into a single kingdom.
Token Minority: Not really. There were probably no black people or minorities in Britain at the time, unless you count things like a Frankish mercenary a minority. You could at a stretch count Severus, who is a Roman of Sarmatian/Alan descent.
Treacherous Advisor: Before the start of the novel's events: Gworthigern. Twice.
Unexpected Successor: As a bastard and third son of a Welsh king, Uther never expected to be king.
Unreliable Narrator: The POV character doesn't always know what's going on, and often portrays things the way they see them rather than the way they really were, or the way the author thought they were. QED calling the Saxons "curs" in the prose.
Vestigial Empire: The Roman Empire collapsed in the fifth century, leaving crumbling roads and aqueducts to lace Europe-but only the Western half. The Eastern Roman Empire
 continued on until the fifteenth century: advanced infrastructure, a 
literate culture, and a higher standard of living, but in an 
increasingly smaller territory.
Villain Decay: Cerdic, only to resurface politically.
We ARE Struggling Together: The Britons, Saxons, Irish and Picts all infight, though usually in petty raiding rather than outright war. Some of the countries celebrate a new king by raiding their "friends" and neighbours.
What a Senseless Waste of Life: One of the main themes of the series. Immortalised in Morgana's wave quote.
Worthy Opponent: Many of the warriors and Kings view each other this way.