This is a mighty castle where the old lords of the surrounding fertile lands lived thousands of years ago. They were a military power, forcing farmers and nearby villages to pay them taxes and become part of their kingdom. Being strategically located, they also created great dams that they could use to deny water to huge swaths of people if necessary, and controlled the only bridges for miles. They used all this to become exceedingly wealthy, allowing them to conquer Borgost and Salt Town. However, only 200 years later, Vale became the massive power it is today, and the Castle on the Bridge swiftly fell. It is now just another place to live that is under the thumb of Vale.
