city_wall is a wall

[Matthias84]

Older cities have sometimes thick old city_walls usually made of bricks like here in Rostock. Would be nice to see that prominent barriers it in the rendering :-)

[cebderby]

Could add barrier=retaining_wall as a wall too.

[westnordost]

For starters, a city wall could be interpreted as a normal wall with a certain default height if no height is defined.