Start working on this as soon as you can. You'll have to run some errands for a dwarf named Voldrik and some old friends from Denerim. Voldrik wants to upgrade the walls to dwarven standards. Pay him 80 sovereigns (gold pieces) to get the work started. After the first story quest, he'll tell you that he needs granite to finish the job. There is a granite deposit in the Wending Wood just to the east of the entrance to the Silverite Mine.
The other upgrade is for your old friends. They'll ask you to find ore deposits so they can make weapons and armor for the soldier's of Vigil's Keep. There are three deposits out there. Two of them follow the main story, the third is from a side quest in Vigil's Keep ("It Came From Beneath"). The iron ore is in Kal'Hirol. The silverite ore is in the Silverite Mine in the Wending Wood. The veridium ore is in the basement of Vigil's Keep.