Many tourists visit Kingsbridge in the English county of Devon to visit its world famous cathedral. However, the Kingsbridge of the novel on which this series is based is entirely fictional and has nothing whatsoever to do with the real Kingsbridge in Devon, much to the disappointment of people who find they have travelled to see a cathedral that has never existed.
Certain techniques used in the construction of the cathedral in this series could be said to be an error being that they were techniques and styles that were not known at the time. This, however, is explained in the novel: Tom Builder was using his own theories of construction that had never been attempted before, as was Jack Jackson, who expanded on these theories whilst journeying through Europe and also invented his own. It is an historical anachronism but the characters have been fictionally credited with the invention of these methods.
The final aerial shot is the fictional Kingsbridge in the present day shows Salisbury with a CGI cathedral combining elements of Salisbury Cathedral and Wells Cathedral, which were the two cathedrals which inspired Ken Follett during the writing of the novel.
The ages of Aliena, Richard, Jack, Alfred and Martha have all been increased from the book for the purposes of this series. In the book Aliena and Alfred are mid-teens, Jack is early teens and Richard and Martha are pre-teens at the start of the book. Ellen has also been advanced in age from mid twenties (she was around 13 when she gave birth to Jack) into middle age.
Although not shown here, Norman castles (castles didn't exist in England before 1066) would have been whitewashed so as to be seen from miles around. They were intended not only as a defensive structure but also as a statement of the wealth and power of the Norman Lords and crown. We only think of castles as bare stone because over time the whitewash has disappeared and not been replaced, leaving the stone-faced castles we see today. If painted white they would look strange to our modern eyes.