Beautiful medium-difficulty route around the source of Río Castril. From the farmhouse of the Hydroelectric Power Plant, we enjoy a comfortable walk parallel to the power station's loading channel. To the left are the terraces of the beautifully cultivated orchards, of which some samples can still be seen. This was undoubtedly an orchard, in another time in which an artisanal subsistence agriculture was carried out. We walk through the valley of the Castril river, flanked by the high cliffs of the surrounding mountains.

The arrival at the Nacimiento del Río is an event that we will always remember. Depending on the rainfall, between 1,000 and 12,000 litres per second, which fall in a cascade, after sprouting through the different cavities. Now we also know, thanks to the latest research, that submerged behind the spring, is a chasm that is over 100 metres deep and that will connect with the large karst cavities of the sierra.

We then ascend gently up to the Osa ravine. Here is a complicated section protected by steel cables. The Osa cave was used by primitive man, but also more recently by shepherds and loggers, making it a historical place.

Shortly after we find one of the most relevant landmarks of the route: the centennial yew, which has undoubtedly been saved from human predation, due to the difficulty in accessing the place.

We now head across gentle hills to the farmhouse of La Puerca, located on a stand of farmland, now converted into a meadow. We must look out carefully for the exit, located to the south and marked by beacons. We will zigzag down a beautiful manmade path among the hillside debris, to access the farmhouse of La Puerca. The descent brings us closer to the beginning, after crossing the Castril River by means of a footbridge.

If the weather is favourable, we will undoubtedly enjoy a lovely field day. As basic precautions, fog, snow and ice must be avoided and above all, a good sense of orientation is needed. If we go with children they must be monitored at all times, since we travel through wild areas with various dangers.