Age of Empires Series Wiki

Set up a blockade on enemy Home Cities, preventing them from getting any shipments!
—In-game description
Your blockade has arrived.
—Message once Blockade is researched
[Player name] has placed a blockade on the Home City of all your enemies!
—Message when an ally has researched Blockade
[Player name] has placed a blockade on your Home City!
—Message when an enemy has researched Blockade

Blockade is a technology in Age of Empires III that can be researched at the Capitol by European civilizations or the Town Center once the Imperial Age is reached by Native American and Asian civilizations. Once researched, it prevents all enemy players from receiving Home City shipments for the rest of the game.

As with Spies, Blockade has a variable cost that depends on how many units the enemy has, but with no minimum or maximum limit.

Blockade is unavailable in Treaty – No Blockade. Except if the starting age is set to Imperial/Post-Imperial Age, it is also unavailable in Deathmatch as players cannot advance into the Imperial Age.