Terry Jones has previously done a series about the barbarians that uses selective and unrepresentative titbits to "prove" that the classical barbarians were actually a million times more advanced than we commonly think. They weren't, but they can be made to appear to be.
This continues in The Crusades where a few out of context points, a wry look and a bit of tired Monty Python comedy is thrown in to persuade the viewer that Muslims were innocent victims and Christians warmongering monsters.
It's pathetic that people are so keen to have their own bigotry propped up that they'll twist history to suit themselves. If this show did the same thing to malign Muslims instead it would probably have a score of around 2. Tell idiots what they want to hear though and you're set for success. Read about the crusades from a history book and you'll find quite a different story. What really galls me is there is little enough historical content on TV, what's the betting he'll get yet another series where he'll spin similar drivel while proper historians don't get a look in.
Really, this is sad stuff though I'm not at all surprised to see it has such a high rating.