Free Skies - Rise of the Runelord
Map of the Fort
If you have means of flight, you could either sneak in from above, or fly to the Eagle’s Aerie, and use a hidden path to come down from the north.
Another option is a cave behind the waterfall. The caverns haven’t been used in decades and are infested with shocker lizards, but they are another means to get behind the defenses.
Near the south gate, there is a sluice gate that releases refuse and sewage. The only problem with this way is that it is near one of the main gates.
Finally, with stealth and means of obscuring sight, you can simply sneak in under the ogre’s noses. They see in the dark, so it wouldn’t be easy, but it is still doable.