View looking across the foundations of the "London Gate",to the depleted remains of the 3rd century town walls.It's worth noting that all across the Empire,any substantial town,which did not already have town walls,had them constructed in about 260AD.A time of great uncertainty.The mighty,all conquering Roman Army had just recently been humiliated by the Parthians,when the Roman Emperor Valerian was captured by the Parthians,causing great instability throughout the Roman Empire.
see http://en.wikipedia.org/wiki/Valerian_I In fairness to Valerian,Rome hadn't fought a field campaign in almost two generations.
see http://www.roman-britain.org/places/verulamium.htm