Anticipating attacks, the London Wall was built around 200 AD to protect the town. It encompassed the town except for on the river frontage, but it was later filled in here as well. The wall was almost 20 feet high and 10 feet thick. This wall fortified London and displays their preparation for expected attacks and determination to keep their land. 

(Image found at http://www.greenbeltsociety.org.uk/history/)