I'm not sure exactly how to go ahead with this:
Arthur isn't a historical figure. There may have been "an" Arthur who eventually metamorphosed via stories into a King, but at best, there MAY have been an Arthur who was a war duke of the britons against the Saxons. There is no conclusive proof of this, however, and most scholars actually remain unconvinced about any archeological evidence about Arthur.
This Arthur would have been sub-roman, which is to say, AFTER the fall of the Roman empire. At the very least, after Rome had no interest at all in the British isles, which would lead most Britons to think that the Romans weren't the best guys to be friends with. Though it is true that many probably imitated and continued Roman culture and ways, they felt no alliance or allegiance to the Roman Empire, which no longer existed in any case.
Note also that the origins of the stories are primarily Welsh, with some influences much more northern, and only a few from the mainland, well after Rome was something people's grandparents were generations too young to remember. Wales and Pictland/Scotland were at best on the fringes of Roman control, often outside of it completely.
The Round table, as such, probably never existed as described, true. But this is mainly because it was supposed to have sat 144, and such a table would never have been moved. It was actually not uncommon for round tables to be used for people to sit at in counsel (there was, however, still a best seat: that near the King, who was a figurative head). If some King in the 5th or 6th century did have a round table, it was probably empty in the middle so servers could approach from the opposite side.
Excaliber is not so simple either. There are about a half dozen theories as to what is meant or what it means. None are truly prevalent or totally convincing. Most actually have more to do with pagan and/or Christian rituals than anything else.
Camelot has several contenders -- if such a place even existed, stretching across most of southern Britain. Not all (in fact most aren't) are seaports or even large villages today.
It should be noted that there is NO mention whatsoever of a King Arthur until several centuries after he supposedly lived. Which means NO primary sources and no way to figure out where the authors of those histories even got their ideas about Arthur. There were 12 great battles associated with either Arthur or one of his predecessors (Uther or Ambrosius Aurealis) were involved in. but even these battles are not mentioned till hundreds of years after they occurred. It should be noted that only 2 of the locations of these battles can be established with any degree of certainty (and those are still contended), and the most important, badon, is totally unknown. There is some evidence that the war these involved occurred, but not enough to be conclusive and there is nothing establishing Arthur as a real figure, let alone the leader of a Britonic empire or collection of kingdoms.
While most of what Spriggan said are valid interpretations, I would warn against assuming that any of them are conclusively true, or even accepted by any majority of scholars.
As for Merlin: change most to "some" and you have something. Myrddin may also have been some sort of Celtic God. He also may have had nothing to do with it, as the strongest argument for the connection rests with a linguistic twist that is possible but highly speculative and unproven.
