abstract
architecture
barbed
barbedwire
barrier
blue
border
boundary
captivity
capture
chain
close
danger
defence
detail
enclosure
environment
fence
gray
horizontal
imprisonment
industrial
invade
invader
iron
jail
knot
marker
metal
military
obstacle
oppression
prison
prohibited
property
protection
secure
security
sharp
simple
sky
strong
symbol
trap
twist
wire