This was a weird but interesting fining, I knew realised how important this bit of "ancient" design was. THE KEY TO A BRIDGE! Is the middle block =O
As explained on the website i found...
The keystone is the most important stone in an arch bridge, without this stone the arch would collapse. The keystone holds the arch together.
Heres the link I found this useful bit of information from^^
Anddd it has examples of bridges in europe =O Big difference to the stone bridges in asia no?