As the closest point to continental Europe and the site of the estuary of the Dour, the site chosen for Dover was ideal for a cross-channel port. The Dour is now covered over for much of its course through the town. In the Roman era, it grew into an important military, mercantile and cross-channel harbour and - with Rutupiae - one of the two starting points of the road later known as Watling Street. It was fortified and garrisoned initially by the Classis Britannica, and later by troops based in a Saxon Shore Fort.