abstract
background
barb
barbed
barricade
barrier
black
border
boundary
closed
concept
crime
criminal
danger
dangerous
dark
depress
depressed
edge
element
fence
forbidden
frame
free
freedom
gray
horizontal
line
mesh
metal
military
monochrome
mortal
murder
no way
no way out
nobody
old
prison
private
protection
protective
razor
safety
security
sharp
sign
silhouette
sky
spiral
stable
steel
stop
stress
wall
war
warning
wire
wired