Places for info on Japanese Siege Warfare?

I've been trying to find details on the ways that the Japanese conducted sieges against castles during the Warring states period, but I can't find anything concrete. I've seen depictions of samurai and ashigaru scaling the sloped castle walls, but that didn't explain how they got over the straight walls at the top of the stone segment. Plus, that seems very ineffective. I'm looking for any good resources you know of - I've already been to wikipedia and that doesn't really do me any good. Books or personal knowledge would be preferable, but websites will work too. I'm really hoping for some solid information - Thank you!