Eager to protect Londinium, the Romans built a wall around the city at the turn of the 3rd century, and kept working on it for the next 200 years. The wall was composed of Kentish ragstone rubble, held together with mortar, and interspersed with bright red stripes of tiles. It was adopted and adapted by later Londoners, until falling into disrepair in the eighteenth century. Today, only various fragments remain.
Some pieces of wall are well-known and substantial; they can be found just outside Tower Hill tube station and alongside the Museum of London, for example. (If you want to explore in much more detail, the Museum of London's London Wall Walk is still available online, although many of the 23 information panels are now damaged or missing.) Other sections are in more surprising places - even an underground car park.
Two pieces of late Roman wall find themselves in another surprising context. On the east side of Jewry Street is the former Sir John Cass College, built in 1902 and currently occupied by London Metropolitan University. Within its basement are the ancient fragments - carefully preserved, in a manner wholly evocative of twentieth-century education establishments, within glass-fronted cupboards. The larger piece even has a label.