Destructible barrier is an object that blocks a choke-point(e.g. a door) that can be destroyed by either player or monsters/npcs.
Usual guidelines:
1.The barrier must not be too hard to destroy, unless its a requirement for a quest/area specific timer. Regeneration/Repair of barrier should be reserved to player made barriers only.

2.Monsters only target the barrier if attacked from the other side somehow, and usually should not engage the barrier(low-priority). Barriers doesn't provide any offensive capability.

3.Barrier has to look like organic feature of the local terrain/building. Barriers that look out of place(to increase visibility) may reduce immersion.