abstract
background
barb
barbed
barbedwire
barbwire
barrier
black
border
boundary
camp
cloud
concept
construction
crime
criminal
danger
detail
fence
forbidden
frame
freedom
grey
guard
immigration
iron
isolated
metal
military
nature
pattern
prison
private
protection
razor
safety
security
sharp
silhouette
sky
steel
symbol
technology
texture
war
white
wire