Their are four types of towers you use can choose between and these can be upgraded as you progress through the campaign. The basic strengths and weaknesses of the towers stay the same, the towers are:
Infantry Tower
Focused on ground units, the Infantry Tower deploys a swarm of dwarves to each map. It’s strong against magic armour and weak against physical.
Cannon Tower
Focused on ground units. the Cannon Tower has slow attack speed but is strong against enemy swarms and fortified armour.
Bow Tower
The Bow tower has a fast attack speed and is weak against physical armour and strong against magic armour.
Magic Tower
The Magic Tower is able to hit ground and air troops with medium attack speed. This tower is strong against physical armour and weak against magic.
Note: The early campaign will auto-Upgrade some towers for you.