Around 50 CE, give or take a few years, a group of Roman military engineers picked a spot on the River Thames and bridged it. They picked the place they did because it was the narrowest practical crossing downstream of the marshes that lined the river's lower reach, with banks workable enough to anchor timber piers and high enough not to wash out at high tide. The bridge they built ran roughly 280 metres across the water on at least nineteen wooden pilings. On the dry, slightly elevated north bank where it landed, an opportunistic trading settlement took root, attracted to the only place for miles where you could reliably get from the south side of the river to the north on foot. They called the settlement Londinium . The bridge has been rebuilt many times. The Roman timber structure was patched and replaced for centuries, then collapsed into disrepair after the Romans withdrew from Britain in 410 CE.…