This quest can appear in either the Imperial or Stormcloak quest-lines depending on whether or not you agreed to hand over the city of Riften to the Imperial Legion during the ceasefire negotiations that took place during the quest Season Unending in the main quest-line.
The quest giver will dispatch you to assist the allied soldiers who are about to attempt to take Fort Greenwall from the enemy. Travel to the Fort, which is situated north of Riften. Follow the trail and you’ll eventually spot it. As soon as you join the allied soldiers, they will automatically charge at the Fort and you should join them. The best approach is to first take down the barricade near the main wall of the fort. When the percentage count of enemies left alive reaches 0% you have taken the fort.