Legate Rikke issue orders to assist with taking Fort Dunstad from the Stormcloaks. Travel to Fort Dunstad and meet up with your fellow legionnaires. Note that there is a giant's camp, Red Road Pass, almost midway in a straight line between the Pale Imperial Camp and the assembly point; take care to circle around to avoid an extra battle.
Approach the Legion soldiers and they will start running towards the fort. Attack the fort with them and dispatch the incumbent Stormcloaks. Take out the archers and destroy the barricades at the fort entrance to allow the Legion to storm in and assist with the rout. Once the enemy percentage counter drops to 0%, the quest is complete.
You may then return to General Tullius for the next quest.
Even though you obtain a shield as the end reward, General Tullius will say "Here, take this blade."
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PC 360 PS3 On meeting the Imperial soldiers, there may be no Stormcloak soldiers in the fort and therefore no way of completing the quest. No fixes have yet been provided, however there is a workaround (see below).
360 PS3 Sometimes Legate Rikke will not offer the dialogue option to start the quest. If this happens wait and try again until the "Reporting for duty" dialogue option appears.
360 PS3 It may happen that when given the quest, no quest marker is activated. Also, even when a quest marker is present, the Imperial soldiers you are supposed to meet will not be present, neither are there Stormcloak soldiers in the fort. It has been tried to play again through the previous Legion-quests in order to fix this, but the problem would happen again. It is best to save before speaking to Legate Rikke and if the marker doesn't appear, reload and complete other quests and return at a later date.
PS3 If the Fort Dunstad quest is bugged, continuing the Season Unending quest in the main storyline will set The Battle for Fort Dunstad to complete. The quest updates when both Ulfric Stormcloak and General Tullius to agree to attend the peace council.
PC If there are no Stormcloaks in the fort and the Imperial troops do not spawn the quest can be completed using the completequest CWFortSiegeFort console command and then getting the quest from Rikke again.
PC Tried using completequest, and it bugged on me. Reloaded a previous save, used setobjectivecompleted instead. Requires several commands, but should work if done right. Make sure to save before any changes are made.
PC One command that appears to work is to type in setstage cwfortsiegefort 10, which resets the quest to the stage where the imperials start appearing to defend the fort. This method avoids you having to mess with the main questline, "Reunification of Skyrim", or reloading an old save file.
PS3 Completing other quests and returning to the fort later may allow the quest to progress.
PS3 After delivering the forged orders and before talking to Legate Rikke to begin the battle, go inside one of your homes and wait 31 days. This is a trick that is used to clear the game world of various clutter (dead bodies, dragon skeletons, arrows, knocked over objects, etc.) which may cause memory issues on the PS3. Upon waiting for 31 days, proceed with the quest as usual.
360 Reload a save from before reporting to Legate Rikke and receiving the quest. Travel to Fort Dunstad and enter the fort, walking around until you have turned the Stormcloaks into enemies. Run around the fort until you have activated all Stormcloaks, and then leave the fort without killing any (Whirlwind Sprint makes for a fast getaway). Run until you can fast travel to Legate Rikke's camp, then receive the quest from her. Fast travel to a location near to the fort (such as the Weynon Stones) and travel to the marker on foot where you meet up with the Imperial soldiers. The fort should be populated properly with Stormcloaks, and can be cleared as intended. (NOTE: This may actually worsen your chances at completing the quest as Rikke will stop offering the "Reporting for duty" dialogue option.