Miller's Gate

From Witcher Wiki

At the end of Act I, the Miller's Gate is the scene of Geralt's ambush by 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) to leave the city of Vizima.