The Mongoliad by Neal Stephenson

A gritty, action-driven historical epic set in the early 13th century that follows a disparate band of knights, sailors, clerics and tradesmen as they confront the advancing Mongol/Tatar threat to Europe; the story mixes brutal battles, sieges and naval engagements with courtroom intrigue, religious conflict and detailed close-quarters swordplay, exploring loyalty, honor and survival as small, resourceful defenders try to hold back a sweeping invasion and protect fragile communities and political orders.