As castles were not simply military buildings but centres of administration and symbols of power, they had a significant impact on the surrounding landscape. Placed by a frequently-used road or river, the toll castle ensured that a lord would get his due toll money from merchants. Rural castles were often associated with mills and field systems due to their role in managing the lord's estate, which gave them greater influence over resources. Others were adjacent to or in royal forests or deer parks and were important in their upkeep. Fish ponds were a luxury of the lordly elite, and many were found next to castles. Not only were they practical in that they ensured a water supply and fresh fish, but they were a status symbol as they were expensive to build and maintain.