First: they drove piles into the bed of the river--not upright piles, but inclined at an angle; they placed two piles side by side, and opposite to these two more; they connected the two piles by ties and the opposite piles with them by transverse girders. Across them they laid a huge beam--a tree roughly hewn, and across these beams they laid the floor of stout planks. The weight of beams and planks and the parapet put up afterwards, with perhaps other planks for greater safety, pressed down the piles and held them in place. To prevent the current from carrying them away, each double pair of piles was protected by a 'starling,' formed by driving upright smaller piles in front at the piers and enclosing a space, which was filled up with stones, so that the force of the current was not felt by the great piles.
In this way the Roman Bridge was built. You will understand it better from the drawing, which shows the Bridge taken from the Embankment near the present site of St. Mary Overies Church. The gate is the river-gate in the long straight wall which ran along the bank of the river. The wall, it is obvious, must have been pierced at several points for the convenience of trade and the quays: one supposes that these posterns could be easily closed and defended. This river-wall, we shall presently see, was standing in the time of Cnut. Some parts of it stood until the building of the stone Bridge in the last quarter of the twelfth century. The Roman Bridge was also the Saxon Bridge, the Danish Bridge, and the Norman Bridge.
In course of time the river-wall was removed, bit by bit: its foundations still lie under the pavement and the warehouses. The gate was altered. I do not suppose there was much of the original structure left when the East Saxons took possession of the City after a hundred years of desertion and decay. But a gate of some kind there must always have been. The breadth of the Bridge allowed, according to FitzStephen, two carts to pass each other. That means about sixteen feet. Like the very ancient stone bridges of Saintes and Avignon, the Bridge was from sixteen to twenty feet broad. The river-gate stood at the south end of Botolph Lane, some seventy feet east of the present Bridge: the second Bridge--the first of stone--stood between the first and third, having St. Magnus' Church on the north and St. Olave's on the south side; together with its own chapel of St. Thomas on the Bridge itself, to place it under the special protection of the saints most dear to London hearts.