The Merchants' Gate, the eastern gate of Vizima, is where Geralt first encountered Mikul, one of the Outskirts' most prominent citizens, at least according to the Reverend. The Merchants' Bridge leading up to it was also the scene of a Salamandra ambush involving Kalkstein.
These gates remained locked for the duration of the game. Although Geralt did see both sides of them, he never actually passed through them.