abstract
background
barb
barbed
barbwire
barrier
black
blue
border
boundary
cage
chain
closed
construction
crime
danger
dangerous
defense
enclosure
fence
forbidden
frame
freedom
gate
guard
iron
isolated
jail
limit
mesh
metal
military
nobody
outdoor
pattern
prison
private
protection
protective
safety
security
shape
sharp
sky
steel
wall
war
white
wire
zone