According to legend, there must be at least six ravens living at the Tower of London at all times, or else the kingdom and the tower itself will fall. This belief has been a part of British folklore for centuries, and it is said to have originated during the reign of King Charles II in the 17th century.
To this day, the tradition of keeping ravens at the Tower of London is maintained by the Yeoman Warders, or "Beefeaters," who are responsible for the care and welfare of the birds. The ravens are kept in a specially-built enclosure within the Tower, and they are fed a diet of raw meat and bird biscuits. The birds are also given plenty of opportunities to fly and exercise, and they are closely monitored by the Tower's staff to ensure their well-being.
It's worth noting that while the tradition of the ravens at the Tower of London is steeped in folklore and legend, it is also a very real and serious responsibility for the Tower's staff. The ravens are considered an important part of the Tower's history and identity, and their welfare is taken very seriously.