This series' strength lies in its ability to immediately transport the viewer to an exotic world. Gorgeously shot in the forests and hillsides of Wales and the Czech Republic, Britannia draws upon ancient Celtic and druid history for its costumes and other design elements. This attention to detail pays off in the way the show is able to draw acute distinctions between the various woodland tribes that populate Britannia, and creates an enormous visual contrast between those tribes and the Romans who have come to usurp them. There's a lot of rich imagery to take in, deployed with plenty of detail and imagination.
Unfortunately, that meticulousness doesn't carry over to the story itself. Characters are often drawn in broad strokes, such as the Roman war-monger, Aulus; the helpless preteen, Cait; and Pellenor's rebel-warrior daughter, Kerra. In fact, Britannia displays a surprising disinterest in characters that do show the capacity to grow or change. For example, one Roman soldier begins to have remorse for the murders he's committed, but then he's quickly dispatched -- turned into a kind of zombie messenger by the druids, ridding him of any hint of the emotional depth he'd previously shown.
With its warring factions in an unfamiliar (and British-accented) land, the obvious touchstone for Britannia is Game of Thrones. But at the heart of Thrones' conflicts are complex characters making choices that have real consequences for the world around them. Without that foundation, there's just not enough investment in Britannia's characters to care about their wars. The spectacle itself is impressive, but the lack of substance underneath keeps Britannia from really landing.