Skyrim:Battle for Windhelm

Quick Walkthrough
#Fight through waves of Stormcloaks.
#Reach the Palace of the Kings.
#Force Ulfric Stormcloak to surrender.

Detailed Walkthrough
For the Empire! For the Legion!
Arrive at Windhelm for the battle to begin. You will find General Tullius at the gates, giving a speech to a group of Imperial Soldiers:

When he finishes his speech, he and his soldiers will charge into the city. Follow them through the main gates.

Battle for Windhelm
Once inside, you will be met by wave after wave of infinitely respawning Stormcloak Soldiers. Your task now will be to fight your way through the opposition to reach the Palace of the Kings. Debris will block the direct route to the palace, forcing you to find another way. After entering the city, take an immediate left and head west to the market square. Upon reaching the square, turn right past the blacksmith's forge and travel north to the cemetery. Once there, take a left down the steps, then take the steps north past the Hall of the Dead. Upon reaching the top of the steps and smashing the barricade, march through Valunstrad and head east down the alley. Once you reach the end of the alley, turn left and head north up the steps, destroy the barricade, and proceed to the Palace of the Kings, where Ulfric and his second in command, Galmar Stone-Fist, are waiting for you.

Palace of the Kings
When you enter the palace, Ulfric will be sitting on his throne with Galmar standing by his side. General Tullius and Legate Rikke will enter, and Tullius will demand Ulfric's surrender, stating that Ulfric is the only one standing between Skyrim and peace and that he needs to be killed to regain the peace. After they're done talking, Ulfric and Galmar will become hostile and attack the three of you. Kill Galmar and force Ulfric to his knees (Ulfric is still essential at this point). If you are having trouble with this battle, you can allow Tullius and Rikke to do most of the fighting, as they are both essential. General Tullius will then prepare to kill Ulfric, but Ulfric will request that his death be at your hands. You will then be given the choice of executing Ulfric or making Tullius do it himself. If you choose to kill Ulfric, Tullius will hand you a leveled enchanted sword for the task, though you don't need to use the sword to kill Ulfric. If you choose to let the general kill Ulfric he will still give you the sword afterwards.

After Ulfric is dead, Tullius and Rikke will head outside. Follow them out and watch as Tullius gives a speech to the assembled Imperial soldiers, thanking them for their service.

After the completion of this quest, the Imperials will install a new Jarl, Brunwulf Free-Winter. If you later approach him, he will grant you the privilege of buying property in Windhelm and, if you speak to him, the opportunity to become Thane of Eastmarch.

Achievements
One achievement is unlocked at the end of this quest:
 

Notes
This quest, Battle for Whiterun (Imperial), Battle for Whiterun (Stormcloaks) and Battle for Solitude are all different radiant versions of the same quest.
During and after the siege of Windhelm, three catapults are present in nearby Windhelm Military Camp. They can be activated to fire projectiles into Windhelm, though this has no effect on the siege.
To minimize game lag, bodies disappear soon after they have been killed.
If you complete this quest before finishing the main quest, you can encounter both Ulfric's and Galmar's spirits in Sovngarde, lost in Alduin's soul-ensnaring mist. Ulfric expresses dismay at how all his war did was feed Alduin, while Galmar is incensed that even in Sovngarde the Dragonborn still dogs his steps. After the defeat of Alduin, Galmar will state that an eternity is too long to hold a grudge and praise you for defeating Alduin.
Finishing the quest will clear any bounty currently held by the player in Eastmarch.
Contrary to post-quest dialogue indicating that you can raid Stormcloak Camps to further help the Legion, the commanders of these camps will still be marked as essential after the quest.
Completing this quest will harm the relationship you have with outspoken Stormcloak supporters across Skyrim, giving you quite a number of hostile greetings. NPC functionality like trading or quest stages will not be affected though.

Bugs
 

 

 
This may be caused by advancing through the Civil War questline far enough that the Season Unending quest can't be started, resulting in Ulfric and Galmar mistakenly maintaining their essential status. Reload from before starting the battle and instead play through the rest of the main questline, or reload an earlier save that will allow you to complete the Season Unending quest. Alternatively, type   in console before entering the palace to mark Season Unending as completed.

 
Reloading a previous save may only work if you go back before the start of the quest.
  This can be solved by opening the console, selecting the gate and using the "unlock" command.

 
  You can use console commands to complete the quest, but Ulfric will continue to rule Windhelm and the house there will be inaccessible. A workaround to complete the quest with full imperial occupation of Eastmarch is to continue with General Tullius and Legate Rikke into the Palace, confront and defeat Ulfric, and then leave the city for thirty days until it resets.
This is possibly because General Tullius will follow his showdown scripting once you get him into the palace. When you enter the palace you need to move quickly away from the door or you will become stuck in cinematic mode. A solution to this issue is to quit without saving, and let the game pick up the auto save from inside the Palace of the Kings. This may take two or three attempts. The other thing you need to be careful about is the presence of two Windhelm guards at the door and Yrsarald Thrice-Pierced. The guards will attack General Tullius and Legate Rikke (who will not respond) and Yrsarald Thrice-Pierced will join the final battle, but since he is an essential character, you won't be able to kill him. Afterward, General Tullius will not give his speech and Yrsarald Thrice-Pierced will follow you around attacking until you leave the city. After the thirty days there should be no difference in result from a normal victory.

 
To fix this, simply stand as close to him as possible, save your game, then close and relaunch Skyrim.

 
  Open console and enter:  . This will start the siege quest.

 
  Use the Unrelenting Force shout on everyone at the dinner table. This will give you a bounty in all the locations where the jarls were from. Fast travel away from Windhelm, then back.

 

 
Completely exiting the game and reloading will fix this.
  This can be fixed by opening the console and entering  .

 
  Using the console, you can manually retrieve the base IDs of the commanders and set them to non-essential.

 
You can still kill Ulfric yourself, allowing the quest to continue.

Quest Stages
Only the quest stages and objectives that are possibly relevant to this radiant version of the quest are shown.