The watchtower is a mandatory building of your town used to collect valuable information on your approaching enemies and your close environment. Although you must have one (and only one) watchtower, it is completely optional to upgrade though.

The watchtower provides the following benefits:

It alerts you when an enemy is attacking you, with an icon clearly visible on the bottom right of your playing screen interface;

It gives you various information on the army that is attacking you. The level and quality of information is greatly improved each time you upgrade it by two levels

It allows you to use the search interface in the Global Map which provides much information about close resource points, invaders, and cities. How far your watchtower can reach depends on its level.