All of the key gates and halls of the Forbidden City were situated symmetrically on the north-south central axis of ancient Beijing to symbolise the emperor's great authority, bestowed by Heaven, and the area where he lived being the center of the universe. The central axis passed through the palace walls at various points, including in front of the Altar of Heaven and Earth, so it could be said that the city was designed around it. The northern gate is where visitors enter the Forbidden City today. It is a huge structure with three sections: an outer wall with 36 guard towers, an inner wall with 24 guard towers, and a middle section consisting only of a royal courtyard with an octagonal pavilion in its center. The southern gate was probably not much different from the northern one in size or design, but it was never used as a public entrance; instead, it served as the main exit route for prisoners going to their executions. There were also eastern and western gates, but they are no longer standing. Inside the walls, the layout of the imperial apartments was identical in every hall except for the one in which they lived. The central axis ran down the length of each hall, dividing it into two parts: one for men and one for women. The emperors' private rooms were on the upper floor, while their audience chambers were on the lower floor.