Could be a little unnerving for those afraid of heights. There are tunnels built into the rock, bridges, and some narrow parts to the path. Although, overall, it is well-maintained and frequently used.
A wonderful hike in the beautiful Picos de Europa National Park in Spain. The trail follows the river, mostly up high (it has its ups and downs). The hike links the small mountain towns of Poncebos and Cain de Valdeon. It is a relatively long hike, so either be prepared to hike back from where you started, or pay for a taxi to drive you around to the other side.