"Ponty" as it's known to the locals, is famous for its old bridge, which was, when built, the longest single spanning bridge in the world. The bridge, built in 1750 by William Edwards (a self taught mason) was so long (45m / 140 feet span) that it took three attempts to get it right. The first, a wooden bridge was washed away by floods, the second, of stone, collapsed during construction because of its weight. The third design was also stone, but much lighter because it had 6 large holes in it... 3 on each side, of diameter 9, 6 and 3 feet. Edwards was paid 50 pounds to maintain it for seven years. In 1857, a three-arch bridge was built alongside to make it easier for traffic to cross the river (the old bridge was a bit too steep).
Alcantara Bridge over the Tagus was built by the Roman, Caius Julius Lacer, for the Emperor Trajan.
“Some of the mightiest and most impressive Roman Bridges, which fall broadly into two groups, are to be found in Spain. On the one hand there is the long low bridge of many nearly equal arches acros a broad river valley. Of this kind are the bridges of Salamanca, Cardova, and merida. As a contrast there is themajestic majestic lofty bridge of few and unequal arches thrown across a wide rocky gorge (H. Shirley 21).”