abstract
backdrop
background
barb
barbed
barbwire
barrier
black
border
boundary
camp
construction
crime
criminal
danger
dangerous
defense
design
fence
forbidden
frame
freedom
guard
iron
metal
military
nature
object
old
prison
private
protect
protection
rights
rust
rusty
safety
secure
security
sharp
sign
silhouette
sky
star
steel
symbol
texture
vintage
war
wire