Sentry Tower

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
Sentry Towers are first available on the Bronze Age as the upgrade of Watch Tower. They are available to all the civilisations of the game, and for some they are the best tower they can get. To build one, it costs 200 stone. For most civilisations, they can be upgraded to Guard Tower at the Granary.

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.