Wherever feasible, the foundations of stone-constructed castles would be erected straight upon the bedrock. The builders would dig a deep and broad trench, then fill it with debris that was packed as tightly as possible to provide a stable foundation. The compacted debris would be the foundation for the wall stones. As soon as this foundation was solid, the top layer of soil would be replaced and the process would be repeated until the entire site was covered. In areas where digging down to bedrock was not possible, the foundation might be formed by packing dirt against an existing structure such as an old wall. During the construction phase, work on the castle would have to cease while the builder obtained permission from the ruler who held the land nearby. Once this permission had been granted, work could continue. It is estimated that it took about a year to build a castle of this type. They were not only used for defense but also served as royal residences. Throughout history, many kings and princes have used their own private castles for rest and relaxation or even as a place where they conducted their business affairs. Some of these castles are still standing today; others have been destroyed over time but many good examples can be found within walking distance of any large city in Europe. The most famous European castle was built near Paris. This magnificent fortress is known as the Château de Fontainebleu and it can be seen from far away because of its huge size and grayish color.