Read actual history. Too many fantasy authors do zero research and their worlds feel completely intellectually empty as a result. A Distant Mirror by Barbara Tuchman is a great starting point if you want a huge overview of 14th century life - black death, church schism, endless wars, nobility, peasant revolts, marauding bandit armies, castles, princesses, it's all in there.

Nothing will make your book suck more than not doing research, and yes, that applies to both scifi and fantasy.