Depending on which side of it you find yourself, this is either the Brama do Starej Wyzimy or the Brama na Groblę. It is one of the last sights that many an unfortunate Catriona victim sees as it is their last stop, after Szpital Lebiody, and before the cmentarz wyzimski.
In Acts II and III, the gate is visible on the dike, but locked for the duration of both Acts. In Act V, Geralt passes through it after a skirmish on the grobla, on his way to find Shani who has set up the szpital polowy to help victims of the fighting which has spilled into the streets.