During the Roman era, the area which is now known as London, was considered Londinium. In order to protect this city, a wall was built somewhere in 200AD. Much of the wall was erected parallel to the River Thames and then engulfed an area that was of strategic importance. During the medieval period, the importance of the wall diminished and the area that fell under the scope of the London city expanded. The wall was solid, had tactically smart bastions positioned and well built towers. If you happen to see dark coloured boulders, pillars or jagged edged structures, it is most likely that what beholds your gaze is a remnant of the once ostentatious London Wall. People run in to these ruins at the most unexpected places along alleyways and foot paths. Inquire from the locals and they would tell you how some hotels are actually built on the foundation of this wall, though the same cannot probably be said about a luxury Mayfair hotel.