New Main Objective: Join the men attacking Fort Sungard
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.
Open the world map and you'll see that the meeting place (the above screen) is located north of Fort Sungard. I don't recommend going to the fort at once, even if you've unlocked that location and could use Fast Travel. It's much better to journey from Rorikstead and then go south on foot or by horse.
When you reach your destination, you should see Stormcloak soldiers who are to take part in the attack (the above screen). Save the game and only then approach them, because they'll set out for the fort without warning.
New Main Objective: Take over Fort Sungard by killing the enemy
Labels on the map: 1 - main fort entrance; 2 - south fort entrance (hard to get to); 3 - northeast fort entrance (hard to get to).
Again, it's a good time to go over the principles of this battle. It differs from the previous ones in that the fighting will take place until you eliminate a sufficient number of enemy soldiers and their "strength" parameter drops to 0%. Nevertheless, don't rely on then fact that initially there will be fairly few of them at the courtyard, because the next ones will be coming from nearby buildings regularly. It's good to remember that and not to position yourself near the doors leading to other locations within the fort. Also, remember not to attack your allies, refraining from using area-effect weapons, abilities or spells, or using them only when you are certain they will hit no one else but the enemy.
There are several ways to enter the fort. Your allies will probably run toward the main west gate (the above screen). Choosing this way, even if you're a heavily armored warrior good in close combat, is not a good idea. You'd have not only to break through several barricades in the way, but also take into consideration massive fire from the surrounding archers.
The second way into the fort is from the back, from the fort's south side (the above screen). As the fort has been built on a very high hill, you can fall very easily by going around it, so I recommend using a horse, which will make the whole climb much more efficient.
The last way in is the small passage in the northeast part of the fort (the above screen). It's a perfect way in for a an archer or a mage. None of the entrances are guarded, so it'll be easy to get inside and take a convenient position to attack.
If your character prefers close combat, start eliminating the Imperial Legion soldiers as soon as you reach the proper part of the fort (the above screen).
Don't forget about the archers occupying the surrounding walls and rooftops; try to get to them as quickly as you can (the above screen).
If your character prefers ranged combat, your first priority after you get inside the fort should be to use any stairs leading to the walls. Using a wide view of the battlefield, eliminate the Legionnaires from a safe distance (the above screen). Pay attention to enemy soldiers trying to strike you, not let them surprise you and force you to fight them in close combat.
No matter the playing style, keep eliminating enemy soldiers. As I mentioned before, look out for foes running out of buildings, and not let anyone surprise you. Fight until enough enemy units are eliminated (the above screen). The remaining Legionnaires will flee and you don't have run after them. The mission is now over and you can continue with the Liberation of Skyrim (main objective: Report to Ulfric Stormcloak #3).