1.
If you describe a building or other place as ~, you mean that it cannot be broken into or captured.
The old Dutch fort with its thick high walls looks virtually ~...
= impenetrable
ADJ
2.
If you say that a person or group is ~, or their position is ~, you think they cannot be defeated by anyone.
The Bundesbank’s seemingly ~ position has begun to weaken.
= unassailable
ADJ