What does Barbicans do in AoE3?

Outpost: build limit +100%; Defensive Building: hit points +20%, damage +10%; Walls: hit points +25%; Fort: build limit +1, becomes available. The game says: "Your Hetman can construct Forts. Enables you to construct twice as many Outposts and increases the hitpoints and attack of defensive buildings."

Home City card

Barbicans

Barbicans (tech ID DEHCExtensiveFortificationsPolish) is a Home City card in AoE3: Definitive Edition 100.19.18309, in the decks of 1 civilization, sent from the Fortress Age. Effect: Outpost: build limit +100%; Defensive Building: hit points +20%, damage +10%; Walls: hit points +25%; Fort: build limit +1, becomes available.

Quick answers (3)

Which civilizations have the Barbicans card?: Poles.
What does Barbicans do in AoE3?: Outpost: build limit +100%; Defensive Building: hit points +20%, damage +10%; Walls: hit points +25%; Fort: build limit +1, becomes available. The game says: "Your Hetman can construct Forts. Enables you to construct twice as many Outposts and increases the hitpoints and attack of defensive buildings."
What is the tech ID of Barbicans?: DEHCExtensiveFortificationsPolish.

Your Hetman can construct Forts. Enables you to construct twice as many Outposts and increases the hitpoints and attack of defensive buildings.

What it does

Outpost: build limit +100%
Defensive Building: hit points +20%, damage +10%
Walls: hit points +25%
Fort: build limit +1, becomes available
Hetman: building rate set to 1

1 further effect (interface, visuals or AI) not listed.

Cost: 1
Sent from: III
Times it can be sent: 1
Game ID: DEHCExtensiveFortificationsPolish

In these decks

PolesIII×1

Source: techtreey.xml and the Home City files, AoE3: DE 100.19.18309 (build 25040513).