Walls: they keep one side from the other.
Some stand alone, strong and durable, others wear with time and weather or a battle or two to show what they are really made of; strong stuff those walls.
Some have holes built into them: doors and windows, through which those on one side can see those on the other. Latched and barred, the doors and windows are primarily to keep one side from being on the other.
Occasionally, the tenant (so called because they live beyond the wall) will allow the guest (so called because they have been invited inside the walled camber) to enter. Only a few are entitled 'guest' status.
Walls have other functions.
They are great surfaces on which to display one's ideals.
They can be used to support all sorts of utilities.
Small, self contained holes can be used as receptacles or repositories, thus referred to as 'post' or 'withdrawal'.
Walls can be of any colour or texture, height or thickness, length or breadth.
Walls carry names, numbers, signs, posters, directions, symbols and even a tap or ash tray.
You can stand by them. walk by them, shade by them, meet by them, talk by them, write on them, climb them, fall off them, look over them, build them or break them down.