The breach in the wall is the de facto west gate out of Old Vizima. It leads to the swamp cemetery and ultimately to the Old Manor. It is not easily attained. Geralt must fight his way there from the field hospital regardless of his choice of path. The only differences being the sort of opponents he meets along the way and the placement of the barricades in his path.
Two individuals who can reliably be found in the vicinity are the Order quartermaster, a merchant and Jean-Pierre. The quartermaster does not buy much, if anything, but he does sell an interesting variety of weapons.