You can build a watch tower or fort anytime you have a general in the field. You can only build watchtowers in your territory. If you don't have anyone in a for it is automatically destroyed. You just have to click the building button when your general is not in a town.

You can't build them anywhere, though. Most spaces work, but some don't, especially in rough terrain. And in case you don't know what eggy_slayer means by the build button, it's a big button in the lower right of the screen.

-Simetrical

When you have a Family Member out in the field, the Build button will enable you to build Forts/Watchtowers, both increase your LOS, but the Fort provides a temporary garrison...