I read Timeline last year, and I really enjoyed it. One aspect I particularly enjoyed was the way that Crichton tried to dismantle common misconceptions about Medieval times.
Recently, I re-read Jurassic Park. My area is paleontology, and I noticed a lot of errors on Crichton's part (not "science marches on" errors, but misunderstandings of biology). So I began to wonder... how much of Timeline even accurate to begin with?
14th century -- Timeline takes place in 1357, not long after the Battle of Poitiers.
And... as with most of Crichton's work, it hit-and-miss. In general, he got it pretty good. He did his research, and he gets a lot of the details right, dropping in lots of set-dressing (richly colored clothes and castle interiors, a passing line about folks in the castle wiping their bottoms with linen, etc.). As you said, he tried to dismantle some common misconceptions.
...which made it an odd narrative choice to have the very characters who expressed surprise at the overturning of those misconceptions be the very people who should have known better--i.e. the historians sent back. He may specialize in medieval architecture, but Chris Hughes has a remarkable lack of familiarily with his time period of study.
(and on a personal note, the fight scenes were in that book are atrocious)