Introduction to the Iron Bridge: The Don Luis I Iron Bridge (Ponte Dom Luis I) is a landmark building in Porto, connecting Gaia and the old town of Porto. The bridge has two floors, more than 70 meters high and 8 meters wide, with a span of 395 meters on both sides. The upper floor is used for subway line D, and the lower floor is used for vehicle traffic. It is still one of the iron bridges with the largest span.