Colchester lies at the head of an inland promontory which emerges from the undulating Essex lowlands to meet the tidal waters of the Colne estuary. On the northern and eastern sides this promontory is clasped by the winding course of the main stream, and on the south by a tributary known as the Roman River which, bending northwards, slightly restricts the otherwise open approach from the west. The eastern end of the area thus enclosed rises commandingly above the sharply-cut river valleys and the seaward marshes, which are of sufficient extent to seclude it from the risks of an open shore. The combined accessibility and security of the site at an early date attracted both the landsman and the sea-going trader, who doubtless unloaded his Gallic cargoes at wharves not far from those still frequented by a modest coastal traffic. It is no longer disputed that somewhere within the natural limits of this favoured region lay the great oppidum of the Trinovantes.