Caesar is described as having fled Britain. In fact he made two expeditions, and reported both as successes. Caesar's numerous enemies never questioned this, even though his general Titus Labienus switched to Pompey's side during Caesar's Civil War.
Romans established protective camps at every stop and were protected by a deep ditch, the earth from which was piled 6 feet high inside the ditch, upon which was a wooden palisade. The camp shown had the earth bank but far too small with strange, obviously machine cut wooden shapes on the top, a barrier which wouldn't last very long in an attack.
The optio is a rank within a century, not a legion, despite the character announcing that he is the optio of the 8th legion. Each century of 80 men has a centurion in charge with an optio assisting him, but no centurions are mentioned.
Every Roman camp built, even marching camps, which were built at the end of the day and demolished the following day whilst campaigning had a ditch, a bank and a palisade fence surrounding it. Not one Roman camp in the series has such a defence.
Cait is shown skinning a rabbit. However, the rabbit is not a native British species. It was introduced by the Romans after they had established themselves on the island.
Ania asks Phelan if his tribe is building an alliance with the Bretons. The Bretons didn't exist at this time, they are the descendants of Celts who migrated from Southern Britain at least 350 years after the Roman invasion of Britain. Both the names Breton and Brittany come from Britain. At this time the area was called Armorica and the people living there were Gauls.
When the girl is at her burnt out homestead, she has a cat. Cats were introduced by the Romans, and since they had only just arrived, she would not have had a cat as a pet already.