"Wall hitpoints increased."
In-game description

Bastion is a technology in Age of Empires III that can be researched once the Colonial Age is reached at a Wall, while European civilizations can also research it at the Church (Mosque for the Ottomans). Once researched, all Walls get +150% hit points.

Bastion is available to all civilizations except for the Sioux, who cannot build walls in the first place.

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