Received from: Automatically after choosing "What ultimately happened to the dragon?" dialogue option during The Dungeons of the La Valettes.
Step 1: Defeat the guards protecting the gate
After solving the case with Aryan, you will head to the gate together with Foltest. After a short conversation with the guard the gate will be opened. The next conversation with Roche will be surprisingly interrupted by a dragon. Immediately after regaining control hide below one of the fences. If you decide to take a walk along the middle of the road, you will die of the fire of the dragon flying above your head (M2, 6).
Slowly move towards the city gate (M2, 7), while eliminating more soldiers on your way. The fight should be hard, even right beside the door, as you have Triss, Foltest and Roche to help you.
Step 2: Cross the bridge to the city
Right beside the gate, its outer part will crumble - Triss will support the falling bricks so that you can move on. Cross the burning bridge and try to be fast, or else you will fall. You will come across the dragon once again and the action will move back to the castle.