In shows like Hercules, history didn't always flow in a straight line, and some stories which would've been great fun, got ignored -- can you imagine how much political hay Caesar could've made, if he had the opportunity to take part in the Trojan War? (given that that War was part of Rome's founding tale)
We fans of The Queen of Swords knew that the series didn't always synch up with reality -- we had some dates (such as 1819), but there were some sets of facts that suggested it takes place in the era of strong hacienda-owners holding the power, and other sets of facts that suggested it was the era when the hacienda-owners had little to no power.
But we were fans. Some of us tried to make both sets fit, and drew stories from the collisions which resulted. Otehrs used one set to the exclusion of the other. And still others ignored both sets in favor of writing codas and episode-fics with real history.
(I say "some" and "others"...in reality, each option was used by all of us)
And that's the fun I see possible in Merlin -- where it doesn't meet historical or mythical facts, we can chose to bring history/myth to visit it (as I recently did with the Backless Maiden, a figure from Arthurian lore), or we can bring them headlong into whatever we want. Where there is no closeness with history or with myth, we can ask "okay, why not?" and grab the truth snake that way.
"The law is the law." -Uther.
"And here, I am the law." -Montoya.
Think of the crossovers - ie, Hercules has met Arthur and Merlin once...but I don't think he's ever met anyone who was younger the second time he met them.
This is the IMDB site for The Queen of Swords. Some of the names will likely sound familiar - Anthony Lemke was recently in "The Last Templar"...Peter Wingfield starred in "Holby City" and "Highlander"...Val Pelka starred alongside Sean Connery in that movie about Camelot....and more.