abstract
background
barb
barbed
barbwire
barrier
black
blue
border
boundary
camp
closed
cloud
crime
criminal
danger
dangerous
day
fence
forbidden
freedom
guard
horizontal
industrial
iron
jail
limit
metal
military
old
prevent
prison
private
protection
razor
safety
security
sharp
sign
silhouette
sky
spikes
spiral
steel
symbol
wall
war
white
wire
zone