Ok, once you speak to Hadvar, its time to sneak into the Fort and rescue the others. Follow the marker and you'll find a trapdoor leading to the Prison. While sneaking, follow the path south, then east, then south again to reach the area where the prisoners are. If you move to the southeast area, you'll find the key to the cells on the table.
Pick it up and open up the cells. As soon as they get geared up, some Stormcloaks enter the room. Meh, nothing to worry about. That should be all of the Stormcloaks in here, so head back north to the marker and out into the courtyard. There are like 15 Stormcloaks out here and they all have a marker over their head, making for much more easy targets.
Kill them along with the help of the prisoners and once you've finished the killing spree, report back to Hadvar and then return to Solitude to speak to Tullius. He promotes you to Legate and gives you a piece of equipment. Now fast travel to Eastmarch or the closest point and head to the next camp.