In the late 3rd and 4th centuries, the east and west ends of the landward wall were joined by a new defensive wall that ran along the river north of the wharves, approximately on the line of Upper and Lower Thames Street. Excavations in 1974-5 uncovered a massive collapsed length of wall at Blackfriars. There were two styles of wall construction: one length in the east was built of ragstone with tile courses constructed in a similar (but less precise) manner to the landward wall but with oak-pile foundations overlaid with a thick layer of chalk; the western length of riverside wall had no elaborate foundation, simply large ragstone blocks rammed into the clay. It had token tile courses on the inner face only and it contained stone blocks of re-used from a late second to third century monumental arch and screen. This stone suggests that it may have been built hurriedly in a time of crisis at a later date in the 4th century.
The lack of consistency in the construction of the riverside wall by comparison with the earlier landward wall, suggests that it may not all have been built at the same time. However, it is known that it was gradually destroyed by river action, which undermined its foundations and by the 10th and 11th centuries it had collapsed.
In 1977 and 1978, excavations within the Tower of London exposed the eastern end of the wall just inside the Inner Curtain wall of the Tower. Here the original wall was either rebuilt later, further to the north, or a secondary wall had been built while the original wall was still standing. The later wall however, did not continue in a straight line to the east, as did the original wall in order to meet the landward wall, but turned south at an angle. The reason for building such a defensive promontory is unclear. Coin evidence indicated that it had been built at the very end of the 4th century.
For further information, see Londinium's riverside wall in Military life.