area
background
barbed
blue
border
boundary
camp
chain
chains
closed
company
concept
crime
danger
dangerous
door
escape
fence
forbidden
freedom
frontier
gate
immigration
industrial
industry
iron
metal
mexico
military
patrol
prison
prisoner
private
property
protection
red
safety
secure
security
sharp
sign
sky
steel
system
travel
two
wall
war
warning
wire