Barrier

1. A fortress or fortified town, on the frontier of a country, commanding an avenue of approach.
2. A carpentry obstruction, stockade, or other obstacle made in a passage in order to stop an enemy.
3. Any limit or boundary; a line of separation.
4. A fence or railing to mark the limits of a place, or to keep back a crowd.
5. An any obstruction; anything which hinders approach or attack.