background
barrier
boulder
brick
build
cage
chain
close
cross
danger
dirty
door
enclose
enter
entrance
exit
fence
freedom
gate
grid
grunge
hard
iron
jail
keep
key
link
lock
mesh
metal
net
old
open
padlock
prevent
prison
protect
rock
row
safety
security
solid
steel
stone
strong
surface
texture
wall
wire