- Report for Duty
With the Stormcloaks, this quest will only be available if the Dragonborn gives The Rift over to the Imperial Legion.
Join the soldiers preparing the assaults
The forces will gather to the Southeast of the Fort.
They are just across the road from the Fort itself. When they are approached by the Dragonborn, they will begin the attack.
Take over Fort Greenwall by defeating the enemy
There are a large number of soldiers scattered throughout the fort. Start with picking off some of the Soldiers on the wall. Go on through the main door carefully.
If you more melee oriented, just head straight for the main entrance and fight through the barricades. There are plenty of enemies to be found inside the fort. While inside, keep track of cover or move with a group of soldiers as they fight through the fort. Having the additional help can be very beneficial.