From the Introduction:
Night has fallen over town and its small square. The windows in the two story brick and mortar houses of successful traders surrounding the square are dark and the doors are locked. Still the small lamps hung over the houses wide doorways provide a little light on the cobbled square. At the centre of the square stands a tall oak, its leaves blocking what little moon or starlight might otherwise reach the ground.
During daytime the oak serves to shade the baron ruling the land around the town, or his appointed deputy if he is abroad, as he holds court and passes judgment onto criminals. At nighttime though all kinds of less than reputable individuals might meet in the tree's shadows, where even the light of the lamps doesn't penetrate, to do shady business.