What is a curtain wall?

Question

I read some information about the forbidden city and it said something about a curtain wall. WHAT IS A CURTAIN WALL??

Answer 1

it is a courtain, that is actually a wall

Answer 2

A curtain wall is a wall that surrounds a building but does not hold up the building. For example, the stone wall that surrounds a medieval castle and its inner yard and associated buildings. It is a 'curtain' wall as it is traditionally runs between towers as a curtain hangs between the edges of a window. See the outer, lower wall of the castle in the link below.