The strength of Hertford as a military position was very great, and depended upon the low marshy ground by which it was almost surrounded, and which was liable to be flooded by the waters of three, or rather four, considerable streams, “flumina non profunda sed clarissima.” Of these, the Lea, flowing from the west, received, a few yards above the castle, the Mimram from the north-west, while a little below the castle the combined stream is swollen by the waters of the Beane from the north, and the Rib from the north-east, the combined volume flowing forward under the name of the Lea. Below, or south-west of the castle, along the course of the Lea, is a wide breadth of lowland, which even now is occasionally flooded, and which in former days must have been an impracticable morass. In the other direction the ground, though built upon and forming a part of the town, is for some distance around but little higher than the meadow, though here and there, elevated 4 feet or 5 feet, appear small deposits of gravel. The castle stands upon the right bank, south-east of the river, and its extreme limit, within the counterscarp or outer edge of its main ditch, included a space something in the figure of an ear, the river forming the shorter side or concavity. In length, north-east and south-west, this space measures 234 yards; its breadth varies from 100 yards to 200 yards, with a mean of 140 yards. The ditch, now almost filled up and in part built over, was about 30 yards broad, and no doubt filled from the river with which it communicated at each end. Within the ditch, taking the line of the old bank, or of the present wall, the area is about three and a half or four acres. The mound is placed on the edge of the river, at the north angle of the enclosure. The bank does not include it, but points to its centre, so that the mound, as was not unusual, formed a part of the enclosing defence. The proper ditch of the mound has been filled up.