Note: If you don't want to play through Awakening a second time, make an extra save after completing the third story quest.
After the third story quest is completed, Amaranthine will be attacked. Head there and you'll be given a choice to make, continue saving Amaranthine or save Vigil's Keep. Choose to save the Keep. Remember the quest at Denerim's gates during Origins? Here you go again. Survive wave after wave after wave of Darkspawn. The last wave will consist of The Herald and the Armored Alpha Ogre. The ogre is beefed up version of the regular ogre and The Herald is a stronger up version of the new Darkspawn generals.