If you don’t have a problem with heights, you may develop one after watching this video of someone walking across Caminito del Rey.
In the words of Wikipedia:
The walkway has now gone many years without maintenance, and is in a highly deteriorated and dangerous state. It is one meter (3 feet and 3 inches) in width, and is over 300 meters (984 feet) above the river. Nearly all of the path has no handrail. Some parts of the concrete walkway have completely collapsed and all that is remaining is the steel beam originally in place to hold it up. One can latch onto a modern steel safety-wire to keep from falling, though it can’t hold much weight. Several people have lost their lives on the walkway in recent years; after four people died in two accidents in 1999 and 2000, the local government closed the entrances. However, many adventurous tourists still find their way onto the walkway to explore it.
Seriously, I actually felt dizzy watching this 6-minute video, and had to stop several times because it was scaring me so much. There are plans to restore it but I think they should take that money and just demolish the trail instead. *shudder*