I pass this extraordinary bridge regularly on the way to and from Barcelona and always give it a second look and a series of thoughts.
(Whenever I see it I always think of a slightly similar bridge, the Stari Most (in Bosnia) that was famously blown up in the war there 20 years ago this November. My memory is of watching an explosion causing it to fall, broken into the river. [See 3:00 at the video below])
“The Puente del Diablo, or Devil's Bridge, crosses from Martorell to Castellbisbal in Catalonia, and its gravity defying structure has a surprising stabilizing device. The bridge was originally built over the Llobregat River in 1283 over a former Roman bridge, the remains of which can still be seen around the current bridge, including a triumphal arch. If it doesn't seem quite right for medieval times, the bridge was actually destroyed in 1939 during the Spanish Civil War and rebuilt in 1965. However, much of it is still the original stone.”
See more photos of the “Devil's Bridge” at Martorell at Atlas Osbscura here.