"Call a handful of Irregulars to defend this colony!"—In-game description
Town Defense is a technology in Age of Empires III: The Asian Dynasties that can be researched by Asian civilizations at the Town Center, which spawns four Irregulars (ranged defenders) to defend the Town Center from enemy attack.
Unlike other technologies, Sentry Uprising is available on each and every Town Center the player has, but can only be researched once as with other technologies.
With the Enlist Irregulars Home City Card, City Defense will be available at Town Centers, which is exactly the same as Sentry Uprising aside of the name, but can be used separately, effectively allowing a Town Center to muster two groups of Sentries at once or mustering a second group of Sentries.