Towers, or bastions, were added to the outer face of the eastern stretch of the landward wall about AD350. Semi-circular in plan, at least twenty of these towers were built at spaced intervals on the eastern side of the wall. The towers seen today on the western wall are medieval.
Those on the eastern side, however, are solid and contained re-used Roman building material, including stone from major public buildings and from fragments of monuments and tombstones in the Roman cemeteries that lay outside the City wall in that area.
These towers were solidly-built to take the artillery machines (‘ballistae’), for firing iron-headed ballista bolts. The towers were probably 8 to 9m high and were built as an additional defence to extend the field of fire by providing solid platforms for the catapult machines.
The outer curving walls were built from ragstone like the city wall itself, but lacked the regular tile courses. The internal structure was solid from the foundations upwards and excavations of the towers have found an array of fragments of various sculptures, most of which seem to come from funerary monuments.
The soldier that stands at the opening of the Roman London Gallery at the Museum of London was found at Camomile Street in one of these towers.
The group of towers, which lie within the eastern section of the wall, extend from Tower Hill to the crossing of the Walbrook stream. Since hitherto unknown towers have been found in recent years, and a convincing case has been argued for more, it is very likely that they were spaced at regular intervals of about 60m along the eastern defences.
The most westerly tower, built partly over the filled-up Roman ditch, is now surmounted by the vestry of All Hallows Church, London Wall. One bastion just north of Aldgate, excavated in 1971, was built prior to the formation of a layer of rubble that contained coins and pottery of the later 4th century, suggested a construction date of about AD350.
Another tower at Crosswall, Vine Street (XWL79), excavated in 1979, gave a clear indication of how they were constructed. The chalk foundations of the bastion were built over the earlier V-shaped ditch. The foundations consisted of layers of ragstone, flints and rubble set in gravel and topped with hard-packed blocks of chalk.
The foundation was stepped down in two stages into the bottom of the ditch. These deeper foundations were evidently included to prevent any subsidence on the unstable ground of the former ditch. In the fill of this tower were the fragmentary remains of a tombstone dedicated to a young girl, Marciana.