Spent like 6 hours in that place, its massive. Like others have said, go up the dragons wing near one of the bonfires. You can jump on its arm and then drop down. The sewers lead to another area which you might want to avoid. You'll know you've gone too far away from the story quest if you get to some huge dudes with axes and swords in the sewers.

I believe there is a ladder you can descend to that is next to a gazibo looking building. look for railing you can jump over that has a small path that leads to a ladder. If you're on the main road coming from the upper area grace you drop down from, its to the left of the huge gate with three knights guarding it. Usually when you approach that gazibo area a knight will blow his horn and an archer will be in the back.