The first installment to feature a multiple armor classification system. Which means that you can customize your assassin in any way you want.
The Dead Kings DLC was made free to everyone as compensation for the game's rocky release being riddled with bugs, glitches, and frame rate issues. People who had bought the season pass also got a free digital copy of a UbiSoft game on Xbox One or PlayStation 4 of their choosing.
The game was in development for 4 years.
This game released parallel with another major Assassin's Creed title (Assassin's Creed: Rogue (2014)). Assassin's Creed: Unity exclusive for Xbox One, PlayStation 4 and PC, Assassin's Creed: Rogue for Xbox 360, PlayStation 3 and PC (later launch).
The parkour and combat systems were completely reworked for this game.