Barrier

1. (Fort.) A carpentry obstruction, stockade, or other obstacle made in a passage in order to stop an enemy.
2. A fortress or fortified town, on the frontier of a country, commanding an avenue of approach.
3. Any obstruction; anything which hinders approach or attack.
4. Any limit or boundary; a line of separation.
Barrier gate ,
a heavy gate to close the opening through a barrier.
Barrier reef ,
a form of coral reef which runs in the general direction of the shore, and incloses a lagoon channel more or less extensive.
To fight at barriers ,
to fight with a barrier between, as a martial exercise.