Even though I write fantasy, I try to be accurate with facts when it is appropriate. There are many things that we use in fantasy that are based on historical periods in the real world, and these can and should be accurate. I’ve written about this before relating to falconry, but it applies to weapons, clothing, and travel as well. For example, if you’re including a trebuchet in your story, you need to remember that it was a huge piece of equipment that was around 60 feet tall. It wasn’t something that was mobile, or that could be worked by a handful of people.
There’s a reason to be sure that you have things correct: there are other people out there who know the information. If you totally mess something up in a novel or story, some people will forgive you. Some will be irritated enough to put the book down. There is also a slight risk that someone will tell you that you have it wrong, either politely or publicly.
When I was a teen I went to a Shakespeare festival in Canada. The group I was with toured the prop and costume warehouse, and one of the items we saw really brought this home. For a play about Saint George and the dragon, they needed a mauled body. The prop people took the time to make sure that it was anatomically correct, just in case there was a doctor in the front row. I’ve remembered that ever since.
(I was also reminded of the importance of fact checking this week, when a museum we visited had two taxidermy raptors mislabeled. I was polite, but I did let them know that their birds were identified incorrectly. There’s no way that I’ll be the last birder to visit.)
If you don’t know the details about something you’re writing, take the time to find it out. Your readers who do know the info will appreciate it.