The city of Cambridge is located at a strategically important location for a town: the furthest downstream that the River Cam could be forded, and the furthest upstream that the river is navigable to seagoing vessels. The flood-prone alluvial flat narrows at the site of the Roman crossing (and later bridge). This crossing point is dominated by a spur of rock running to the North-West. This outcrop, which is steep-sided to the river, has been the site of fortifications from Roman times onwards, providing military control of the crossing. Even today, the site is the location of the County Hall, giving continuity of purpose for nearly 2000 years.[1]