How should I handle the dispute over walkway ownership and threats?

Hi, I own a northwest flat with a 22-foot walkway in front of my door, with no other flats in that walkway. In the plan layout, this walkway is marked as a balcony. When I purchased the flat, the builder informed me that it belonged to my unit. However, the adjacent flat owner is now claiming that it is a common area, and the builder is stating that both units must share it 50/50. The adjacent flat owner is threatening me and insisting that I remove the gate and grill. How should I handle this situation?