Time to take out another fort. Fort Hraggstad is just north of the war camp, so you'll need to head that way to meet up with the troops. It's a ways off, so be ready for a little climb. Once you meet the troops, this turns into the exact same type of mission as the other Fort missions. So instead of fixing what isn't broken, here's what I wrote in the last two missions:
With that said, its your job to go on a killing rampage. You'll have to storm the front of the fort first, where barricades will be in place with quite a few archers on the ramparts, but after that it's an all-out killing orgy. Kill as much as you can as fast as you can. Once you've brought down that 100% to 0%, the mission will be complete. There may STILL be stragglers though, and you can hunt them down as well.
It's a fairly easy (but fun!) mission since you have so many allies and can easily pick off Imperials when they aren't focused on you. Or, alternately you may wish to be in the thick of the action, as you can save the lives of some fellow soldiers if you are.
After the fighting, you'll be done with this mission. However, instead of needing to go back and talk to Ulfric, you need to go back and talk to Galmar. Ask him what else needs done, and he will tell you that the time has come to take the city of Solitude. This will start the quest "Battle for Solitude".