army
barbed
barbwire
barrier
blizzard
border
boundary
cold
concrete
danger
defense
escape
fence
field
forbidden
frost
gaol
guard
iron
jail
line
locked
low temperature
metal
military
nature
no entry
perspective
prevent
prison
private
protected area
protection
safety
seamless
season
security
sharp
snow
snowdrift
spike
steel
strategic object
thorn
wall
war
warning
weather
winter
wire