abstract
background
barb
barbed
barbed wire
barbwire
barrier
black
blue
border
boundary
cage
camp
chain
closed
crime
danger
dangerous
escape
fence
forbidden
freedom
guard
iron
line
mesh
metal
military
nature
pattern
prison
private
protect
protection
rusty
safety
security
sharp
silhouette
sky
steel
sun
symbol
system
wall
war
warning
white
wire
wired