At the end of Chapter I, the Miller's Gate is the scene of Geralt's ambush by the City Guard, thanks in no small part to Mikul. That is the only time Geralt passes through that gate, albeit involuntarily. Once inside the city, and free from his unexpected stay in the dungeon, Geralt can no longer pass through this gate (nor in fact any other gate except the one to the Dike) to leave the city of Vizima.