BUILD: Look at the Invader Card in the Build space: the Invaders Build in each land of the shown type only. Whenever there are Invaders in the specified type of land, they add either one City or one Town:

If the land has more Towns than Cities, add a City. (You are not upgrading the Towns, you are adding a new City from the supply.)

In all other cases, add a Town.

Invader Cards do not cause Build Actions in lands without Invaders.