Well, first of all:
Homin8r that's what I said.
Anyway, near the courtyard, there's a small strip of land in between the bridge and the wooden shack. Go up to the wall (it has one square window/hole in it). Start running this. I find it best not to sprint and never stop moving. Also, push the analog stick against the wall while moving. Eventually, you'll be inside the wall. Turn around and start walking towards the barrel that's on it's side. Then start sliding to the right, and you should drop under!
Note: This is just a small part. This goes way further. However, it's to complicated and difficult to explain. Add me if you want to learn the deeper parts, or if you simply have any problems. My name is CHIKABLAM.
Also, when you're adding me, could you just tell me why.