The Maribor gate is described as a southern gate to the city of Vizima by which the lake may be accessed.
The Witcher
The Maribor gate dominates the view to the north of the main village in the Outskirts, but offers no easier entry into Vizima than any other gate. The two city guards flanking the gates doggedly stick to their script: show your pass to Mikul, or get lost.
From the Trade Quarter, this gate is no more passable. From inside the city, the gate is also guarded and even more heavily, so I guess leaving is frowned upon as much as entering.