New Main Objective: Join the men attacking Fort Hraggstad
Note! You get this mission during the Liberation of Skyrim main Stormcloak Rebellion Quest and its completion is required to advance to the next part.
Start by opening the world map. It probably won't surprise you that the Stormcloak soldiers are stationed some distance away from Fort Hraggstad (the above screen). It's not too far from the Stormcloak camp, so you can got there on foot or by horse, heading north.
When you reach your destination, look around for the Stormcloak soldiers (the above screen). Save the game and approach them, "encouraging" them to commence the attack.
New Main Objective: Take over Fort Hraggstad by killing the enemy
Labels on the map: 1- main fort entrance; 2- north fort entrance; 3 - suggested entrance for r combat.
The battle has all the characteristics of a "standard" fort invasion. You need to fight until enemy forces are sufficiently weakened. Remember not to stay close to the doors, because that's where enemy soldiers will be running out from. Don't use area-effect attacks or spell so as not to accidentally hit your allies.
Just like before, the game will allow you a free choice as to how you want to get inside. By default, the Stormcloak soldiers go for the northeast entrance (the above screen), but for obvious reasons it's blocked by barricades and guarded by archers.
A much safer way into the fort is to go around it to the north entrance (the above screen). It's the best choice for a mage or an archer who attack from a safe distance.
If your character prefers close combat, you can attack the Imperial soldiers once you're in the courtyard (the above screen). Just like with similar quests, don't forget about archers massively occupying the upper walls and rooftops. Try to take them out as soon as you can, so you won't be loosing your team members.
If your character prefers attacking foes from a distance, try to sneak into the fort and find a good spot to shoot at them. My recommendation: a tall tower in the north part of the fort (the above screen) with stairs leading to the top. Choosing the tower is especially advantageous because it's located right by the alternative entrance to the fort.
No matter the tactic, try to eliminate enemy soldiers as they come, because the game will be sending them your way quite reguralry. Fight until you receive information of the victory. Traditionally, you can ignore the fleeing forces (the above screen). The mission is now over and you can continue with the Liberation of Skyrim (main objective: Liberate Haafingar).