Walled describes a place that has a wall built around it — a walled garden, a walled city, a walled courtyard. It is an adjective and it goes in front of the noun; you would not usually say "the garden is walled" in ordinary speech, though it is possible. The word carries a feeling as well as a fact. A walled garden sounds protected, private, warm, a little secret — old English houses had them because the wall keeps the wind off and holds the sun's heat, so fruit grows better inside. A walled city sounds old and safe against enemies, because the wall was for defending it. So "walled" is mostly used about older things; you would not describe a new housing estate with a fence as "walled". You will also meet it inside longer words: high-walled, thick-walled, and the useful phrase "a walled garden" now used about computers and phones, meaning a system a company keeps closed so you can only use its own apps and services inside it.