A more thorough explanation:

WALLIA

Wallia is a historical term that refers to a wall, usually a mound or bank, built in marshy areas to protect against the sea. It is also known as a seawall.

One example of a wallia is the Hadrian's Wall in England, which was built by the Romans to protect against the Picts and Scots. Another example is the seawall in the Netherlands, which protects the country from flooding caused by the North Sea.

These examples illustrate how wallia was used in the past to protect against natural disasters, such as floods and storms. They also show how wallia was an important part of defensive strategies used by ancient civilizations.