Having reported to Ulfric Stormcloak, it is time to return to the camp. There you will once again meet with Galmar Stone-Fist. He will direct you to aid his men attacking another of the forts in the area.
Join the men attacking Fort Hraggstad
The Stormcloak's forces have gathered to the East of Fort Hraggstad. As soon as you have joined up with their group, they will draw their weapons. The assault begins just after that.
Take over the fort by defeating the enemy
Once again, as you fight through the Imperial forces, periodic updates will let you know the remaining forces that you need to kill. Once again, you will need to go over the whole of the area and clear out as many of the Imperial Soldiers that you can. Follow the men as they head to the North to reach the main entrance of the fort.
Approaching the fort will have you confront their defensive lines and some archers on the walls above the entrance. If you reach the entrance first, there will be barricades. These are very easy to overcome with just a few swings from any weapon or a quick spell burst. Enter the courtyard carefully. There are a lot of soldiers there and on the walls above you. Clear a path to either side with a ramp and go on up it.
Use the ramp to ascend onto the walls. This will reduce the number of enemies that can reach you and make it easier to quickly deal with the Archers on the walls themselves. Take your time going through the fort.