Brittania on Amazon

Maybe I am late to the party on this one.

The actor who played the Emperor in Star Wars?
The actor who played the Governor in The Walking Dead?

Warring tribes of Britain and a Roman legion?

Super freaky Druids? (And the lead druid is played spookily/convincingly by the actor who was comic relief with the fake eye in Pirates of the Carribean.)

Oh, how could I forget? The queen of one of the warring factions is the broom instructor from Harry Potter.

Maybe halfway through the first season. Pretty entertaining historical fiction.