The Sentry Tower is a defensive structure available on Age of Empires and Age of Mythology, though they are very different in both occurrances.
Age of Empires
See Sentry Tower for more information.
Age of Mythology
In Age of Mythology, Sentry Towers perform a similar role to the Outposts of Age of Empires II. They do not attack, and are used purely for their decent line of sight. Unlike the Outpost, however, they can be upgraded to the Age of Mythology's version of Guard Tower.