Zurbarràn is a small castle in Toussaint, located on the hill near Pomerol castle. It is known for the uncharted network of caves which Geralt of Rivia had undertook to explore.
Spoiler warning: Significant plot details follow.
By coincidence, he discovered the secret meeting of Nilfgaardian opposition plotting against the Emperor and learned Vilgefortz' whereabouts.
Significant plot details end here.