Goofs

Although Julius Caesar managed to beat Casivellaunus's coalition and demanding tribute and hostages from them, he never conquered Brittannia as a whole. Even his victory against the southern tribes was short-lived. Permanent Roman occupation in Brittannia started about a century later, under emperor Claudius.

Caesar seems to be conducting the war from a palace. As they have been in Brittannia for a few months at most, how could the Roman have built this so quickly?

When in the small boat with the barrel of magical potion and his companions, Obelix could have row at full speed to escape the Romans the same way he pulled (or pushed) easily the boats full of stones for the Pyramids in a previous adventure.