Alternate History is an interesting genre. In ways, you could see a lot of Steampunk as an alternate history sort of thing. Weird War II is another that's pretty popular. Dreamlord Press has created a new alternate history role playing game. Written by Davide Pignedoli, City of Judas gives a new spin on the Crusades and introduces us to a Jerusalem infiltrated by the Cult of Judas.
The mechanics of the game are based off of the Apocalypse World rules set, but with some changes to focus it on a Medieval setting. A decent change from the regular is the Advantage Die to accumulate advantages in stead of getting just a +1.
There's a GM section as well, for the person who'll be in charge of the game, complete with a set of guidelines for the setting and a set of instructions on how to create missions. Plus, enemies: humans, monsters, and demons. Because everyone needs a good demon to skewer.
The book is available now.