This river is like a natural fortress which protected the town on three sides, the fourth side of the city walls, and bridges built that can fly up and down.
The river is located in the protected natural consequence the fortress city of the three sides, the fourth side, the city has created a bridge wall and can pull up and down.
The river is like a natural defense against the city and the three sides for the inhabitants of the four walls and bridges built across the can draw up and down.