Suzhou, like many other Chinese cities, was once surrounded by a ring of walls and gates. But since much of the city's commerce and transportation took place on its network of canals, completely walling of the city would have been impractical.
As a result Suzhou created a system of "water gates" - small forts which had openings for both land and water traffic. Just as the wooden doors could close off roads into the city, the canals could be sealed from above. One of these gates is still preserved even today - the Pan Men, one of the only such places in China. Dating from 1531, the gate is not just an opening in the wall, but a small fortress with walls, gates, and guard towers.
It also connects to a section of the historic city wall, great for a quiet stroll wand to see the life of the city unfold beneath. From it you can look out over the bustling activities of the narrow streets and alleys inside the wall, and see small boats plying the canal pass through the water gate, evoking the days when flocks of boats crowded into the thriving canals of China's Venice.