You can work on this Trophy as soon as you freed the Keep and are free to roam it.
The whole thing consists of two parts:
1. Helping Voldrik to fortify the Keep itself
2. Getting the guards some good equipment
Fortifying the Keep
Talk to Voldrik near the Keep's gate. He wants to upgrade/repair the walls to dwarven standard and asks 80 Sovereigns for it. Pay him (you should have enough coin if you imported a character from the main game). Talk to him again after you did your next main quest. He'll need some granite to continue. You can find it in the Wending Woods next to the raided caravan pretty much at the beginning of the map. Return and tell him where to find it.
Equipment for the guards
Herren and Wade have a smithy in the Keep's courtyard. They are looking for ore deposits to make quality gear for you. There are three deposits total you'll have to find.
Veridium Ore: You can find it during a side quest called "It comes from beneath". Talk to Sergeant Maverlies to start it. She's waiting in the courtyard in front of a smaller house. The side quest consists of two parts. It'll be halted pretty much in the middle because your workers will have to clear some rubble. Do another quest and they should be done when you return. Continue and you'll find the deposit in the last room with Adria.
Iron Ore: This one can be found during the main quest in Kal'Hirol. It's in the Trade Quarter at the Forge.
Silverite Ore: It's in the Silverite Mines. You'll also come here during a main quest. The deposit is in the room with the two experimental subjects.