Towers provide the Resource "Guarding". Guarding is helping you to raise your Loyalty. If you have 5 Guarding "Points" then you get 5 Loyalty so its a ratio of 1:1. There are 2 types of towers. The First Tower you get is the "Primitive Tower" and its the only tower you can build directly on the Ground. The other 4 towers cant be build directly on the ground. You need to build Platforms and build them on these Platforms.