SPOILER:

Go back to the area with the soldiers guarding a bunch of prisoners. Near there is a dead soldier in an alley. Use your Aard sign to remove the wall blocking the alley.

Go back to the lpace with many citizens standing outside, praying for their lives (not the ones in the building, the ones outside). There you should find a boarded up passage, tat you can clear with the Aard sign.

Edit: ninja'd! By the Dungeonmaster himself

you need to use the aard sign to blast away a wooden wall. it's in one of the earlier areas, right where a bunch of soldiers and villagers sit around.

edit: ninja'd twice :D

I'll try to keep this spoiler-free:

Down a few ramps from where you've regained control of Geralt are a bunch of Temerian soldiers and their prisoners; look for blocked entrances in that immediate area. It should be something only a magic user would be able to clear, but not the usual debris.

le: wow, thrice-ninja'd.

Thanks a million guys, I'm almost embarrassed at how simple the solution was.