This design is now used widely as the badge of the town, including on lamp posts, and is also incorporated into the town’s coat of arms.
In medieval times, Folkestone was part of the Cinque Ports (a group of towns on the south east coast of England) that provided warships for the king to protect England in case of invasion) and were given trading privileges in return.
In the days before cannons, medieval warships had wooden castles at the bow and stern, from which soldiers fired arrows at enemy vessels.
Here’s Folkestone’s medieval ship replicated in stained glass.
Fascinating fact The silk banner was made to be carried in civic processions and at special events to represent Folkestone Town Council. It appears in a well-known WW1 painting - The Arrival of the Belgian Refugees by Fredo Franzoni (See World War 1 topic 1).