This is a prequel to Fate/stay night (2006) and Fate/stay night [Unlimited Blade Works] (2014), which chronicles the events that led to those series.
Out of all the servants, Assassin, Rider, and Caster are the only ones that were real people (being Hassan-i Sabbah, Alexander the Great, and Gilles de Rais, respectively)
The series was initially planned to be one 26 episode series but producers decided to split it into two 13 episode series.
Caster's true identity, Gilles de Rais, is that of a French army soldier in the 1400s who was hanged for murdering young children.
The second season will be the first time composer Yuki Kajiura will have written the score as well as the opening and ending theme songs, since her work on Madlax (2004), in 2004.