All the filming locations (eg Shrewsbury Abbey, street scenes in Shrewsbury, banks of the River Severn) were sets constructed on waste ground behind Fot Studios in Budapest, Hungary, because almost every medieval building that still exists in the UK is now in a ruined state, due to Henry VIII's Dissolution of the Monasteries in the 16th Century.
Several of the names and occupations of the monks in the series are actually from a record book from the Abbey of St. Peter and St. Paul in Shrewsbury, England. Some of these include Cadfael, Robert, Jerome, and Heribut.
The original choice for the part of Cadfael was Ian Holm; Holm accepted but ITV took so long to bring the project to fruition that he decided to take other roles that were offered to him in the meantime. Philip Madoc, who had played the part in the BBC Radio 4 dramatisations, was also considered.
Brother Oswin, the novice and Cadfael's apprentice, has a much larger role in the television series than in the books. In the books he does not appear until the middle of the series. He largely takes the place of Brother Mark, who left the Abbey to run the leper house at St. Giles. Mark can be seen in "The Leper of St. Giles". In "The Virgin in The Ice" Brother Elyas was the brother accused of the rape and murder of Sister Hilaria, not Oswin.
In the series, Jerome is portrayed as being much younger than Cadfael. Cadfael's age is around 70 and Jerome appears to be in his mid-thirties. In the book "The Devil's Novice" it is stated that Jerome was 20 years younger than Cadfael. It also stated that Cadfael was 60, meaning Jerome was 40. Also, Jerome comes off as naive concerning the ways of the world. In reality, he was quite knowledgeable about the world beyond the Abbey which was what actually set he and Cadfael against one another so often.