Oldrim version here: https://www.nexusmods.com/skyrim/mods/83980?tab=files

Update: Added ESL version

While playing ESO, I noticed that the second era Fort Greenwall has an extra wall that extends to the cliffside. I really liked the look of this, and it makes the only ways to enter Riften from the North is through Fort Greenwall, Climbing the mountain/hill/whatever to the West, or walking all the way around the lake.
So, in a day I put this together. There isn't much to loot (unless you really like empty bottles), and there are no interiors. It is mostly there for looks at the moment. I also left a large hole in wall so It wouldn't block the road to Riften. It still seems to work for NPCs.

In the future, I may make occupied variants for the Stormcloaks, Imperials, and Bandits. Mostly depends if there is interest, I feel like it, and if someone makes a navmesh for the walls.

Issues:
No doors. I couldn't find the right non-usable doors.
NPCs cannot walk atop the walls. But they can walk around them.

Conflicts:
Haven't seen any.

Help Wanted!:
[UPDATE: Thanks to AndrealphusVIII, we now have a Navcut version, thank you!] 
NPCs still cannot walk atop the walls. If anyone wants to help, be my guest!

Future Plans:
Faction versions? (Bandit, Stormcloak, Imperial)
Hole-less version?
Interiors? 

Permissions:
If anyone wants to add onto or do anything with this, go ahead! Just give me some credit! I'd love to see it!

Credits:
AndrealphusVIII, for making the Navcut version! Thanks a ton!
Here is a link to his mods, check em' out!: http://www.nexusmods.com/games/users/5646623/?tb=mods&pUp=1
Me I guess?

Special Thanks:
The Nexus- for hosting this!
AndrealphusVIII- Again for the Navcut version!
OrganicView on Reddit- For helping me figure out the porting process!