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 run 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 run links the small mountain towns of Poncebos and Cain de Valdeon. It is a relatively long run, so either be prepared to run back from where you started, or pay for a taxi to drive you around to the other side.