Is there a way to get up the assassins stonglehold?
I like to know this as well.
I seriously doubt it. I've tried multiple ways up, from the inside and out. From the front of the castle and front the back. The multiple platforms up on the walls seem to be only to deceive you, there doesn't seem to be a way up, or at least from my experience.
I found one spot that looks likely to be climbed but you would have to climb over the cliff-side fence in the castle garden and the game wont let you.