The stretch of town wall to the north west of the London gateway still survives partially intact to a height of over two metres. Like the London gateway it is a flint and cement construction, with layers of brickwork providing stability through the structure. The wall was originally covered with ashlar masonry, but this regular brick was no doubt robbed away for house building after the abandonment of the town. The core of the wall, which you can still see here, seems to have largely survived all around the town until the twelfth century, after which it was gradually robbed for building materials by the monks and townspeople of St Albans. The south western side – furthest from St Albans - was still largely intact in the 16th century, now only this section still survives to any height.

In all, the wall enclosed 201 acres (81.3 ha) of land, making its total length in excess of 2 miles. As it would have probably stood to a height of some four metres, over 66,000 tons of flint and tile would have been needed for its construction. In addition to the wall there was a bank behind it some 15 to 18 metres wide. The section behind the standing wall in this area still survives to an height of some two metres. The earth for this bank came from a ditch built 6-7 metres in front of the wall, meaning that the town defences consisted of a ditch, wall and bank. Along the surviving section of the wall there are also two or possibly three towers projecting out. No towers were built elsewhere on the wall, and their presence on this section, along with what seems to have been an extra deep section of ditch in this area (still visible beyond the other side of the path that runs along the wall) seems to have been to impress travellers approaching the town from London. The wall was probably more of a public monument than a military defence – its size and positioning downslope of the valley top makes it unsuitable for this, although it was certainly enough of a barrier to prevent access for anything less than an organised army, and would allow control of all the routes into the town.

The wall appears to have been built around AD 275, at a time when walls were being built around other roman towns. The population of Verulamium may have been following the crowd in constructing a new town wall, but unlike other roman towns, it may have already been defended.