Fence required. There shall be erected and maintained a fence approved by the Superintendent of Buildings not less than four feet in height, with posts embedded in concrete sunk in the ground to a depth of not less than four feet at intervals of not more than eight feet, enclosing the entire portion of the premises upon which such pool shall be installed and entirely surrounding the area in which such pool is located, except if the sides of the pool are at least six feet in height and entrance can be gained only by use of a removable ladder or a ladder capable of being retracted so that the bottom of the ladder, when retracted, is at least six feet above the ground.