I'm not so sure how looking at Assassin's creed could help, being as it is very firmly rooted in the 3D world when it comes to movement. If you want something like AC's "reach out and grab holes" system, you could look at QWOP's sequel "GIRP" - it's probably too gimmicky to copy for a more traditional game, but the core idea is there. Aside that, Creed had a lot of gap jumping, grabbing onto ledges and shimmying, all of these could probably translate well to 2D in their own style. Prince of Persia might be a good source of inspiration as well.
Also I definitely recommend looking at Super Metroid and the "metroidvania" genre in general if you're looking for traditional side scrolling "free running". There are tons of implementations of wall jumping, trick jumps, etc.