If you have ever been near to the Tower of London or elsewhere in the City of London, you might have seen the remains of a stone city wall which the Romans built to defend the town, perhaps at a time when there were fewer soldiers based in London. The wall was solid, as much as 6 metres high, built from shaped blocks of stone held together by mortar (like cement) and added layers of flat clay tiles. The wall lasted until after the Romans and was repaired and made higher during later periods when people continued to live inside the city wall because they felt safe. This made the ground level inside the wall much higher than outside.

Where the fort already existed, the city wall was joined onto two of the corners of the existing fort. There were gateways in the wall which linked to the main Roman roads that led to all parts of the province. These gates were later called Aldgate, Bishopsgate, Newgate and Ludgate and these names still remain today. The Romans also added Aldersgate at a later date. The gateways allowed people to come and go but they were shut at night.

As an additional protection, the Romans then built a wall along the river, afraid that the town might be attacked by enemy armies sailing up the River Thames. They later added semi-circular towers onto the eastern part of the city wall. Each tower was built to take a heavy catapult machine (ballista) that could fire iron bolts some considerable distance.