Walls like walls you could put in your city, i.e you could add more walls around your prison, you could even in close your whole city with walls of various sizes and colors.
Wall: noun
1.
any of various permanent upright constructions having a length muchgreater than the thickness and presenting a continuous surface exceptwhere pierced by doors, windows, etc.: used for shelter, protection, orprivacy, or to subdivide interior space, to support floors, roofs, or thelike, to retain earth, to fence in an area, etc.
2.
Usually, walls. a rampart raised for defensive purposes.
3.
an immaterial or intangible barrier, obstruction, etc., suggesting a wall:
Is that specific enough or should I get the crayons out? I can be rude too just like you.
a wall of prejudice.
4.
a wall-like, enclosing part, thing, mass, etc.:
a wall of fire; a wall of troops.
5.
an embankment to prevent flooding, as a levee or sea wall.
6.
the Wall. Berlin Wall.
7.
the outermost film or layer of structural material protecting,surrounding, and defining the physical limits of an object:
the wall of a blood cell.
