There was a huge wall around the city centre to keep the intruders away. Inside the walls of Londinium, there were houses, temples, shops and markets. (Nowadays this is the business area called the City. There are no houses to live, just office buildings, skyscrapers and historical sights.)
The business area of Roman London were the Basilica and Forum, they stood where Gracechurch St is today in the City.
You can read more about Roman times in England here.
an intruder (n.) – someone who enters a place where he can’t go, especially to steal or rob
City, the – London’s business area
skyscraper, a – a really high dwelling (= building), the tallest ones in London are The Shard and The Gherkin