abstract
background
barrier
boulder
build
bw
cage
chain
close
color
cross
danger
dirty
door
enclose
enter
entrance
exit
fence
freedom
gate
green
grid
grunge
hard
iron
jail
keep
link
lock
mesh
metal
net
old
padlock
path
pattern
prevent
prison
protect
rock
rusty
safety
security
steel
strong
surface
texture
wall
wire