Structural/Architectural:
* Crumbling wall: This is the literal opposite of a pillar, which is a strong, supporting structure.
* Foundation: While a foundation supports a building, it's usually beneath the structure, whereas a pillar is a vertical element.
* Weak point: This is a metaphorical opposite, as a pillar represents strength and stability.
Metaphorical:
* Betrayal: A pillar of support can be betrayed, so this is a strong opposite in a metaphorical sense.
* Weakness: As a pillar represents strength, its opposite could be weakness.
* Failure: This is another metaphorical opposite, as a pillar represents success or achievement.
To determine the most appropriate opposite, consider the specific context in which you are using the word "pillar."