If you choose to race Ciri across the ramparts, follow her lead and climb the ladder ahead of you. Follow the path indicated via objective markers and make your way up the scaffold. Climb from the scaffold onto the castle wall, now on your left, then drop down to the next level and make a U-turn. Climb down the ladder ahead of you on your left, then down the next ladder at the bottom of the stairs ahead of you. Turn right at the base of the ladder and jump across the gap in the stone walkway, then continue along the wooden scaffold to your right. Follow the stairs downwards until you reach the final ladder - climb down it before Ciri does to win the race.