Where does Laia go after her grandparents are killed in the novel, An Ember in the Ashes?

After Laia's grandparents are killed and Darin is taken away to be interrogated, the Mask tells her to run and she does. She runs through the cramped streets and dusty markets of the Scholars' Quarter to a grate that leads into Serra's catacombs that snake beneath the city. She lifts up the gate and goes below.