Tower

Age of Empires

 * Sentry Tower
 * Guard Tower (Age of Empires)
 * Ballista Tower

Age of Empires II

 * Outpost (Age of Empires II)
 * Watch Tower
 * Guard Tower (Age of Empires II)
 * Keep
 * Bombard Tower

Age of Empires III

 * Outpost (Age of Empires III)
 * Frontier Outpost
 * Fortified Outpost