The one on the left is obviously the most recent - a road bridge built to take traffic around the side of the castle, this was built to replace the central road bridge, built by Thomas Telford, one of the first wrought iron suspension bridges ever built. There was no room on the castle side for the suspension piers, so a bit of the castle was knocked down and the fake "gateways" were built to support the bridge. Then along came railway pioneer Robert Stephenson, to build the bridge on the right, which is tubular construction, built to take the weight of the trains. Both the Telford and the Stephenson bridges are now "listed buildings".
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