There are over 20 different enemies in the game, most of which you will encounter in the early campaign. Below is a brief guide to the four most common foes you will meet:
Gobin
The most common foe, Goblins are unarmored and vulnerable to ALL towers.
Wargs
Wargs are weak to Cannon Towers and travel in packs.
Orc Raider
The Orc Raider has heavy armour that can be countered with a Magic Tower.
Orc Berserker
The Orc Berserker is weak to everything but is able to sustain a large amount of damage which makes them difficult to take down. This enemy is slow which makes Cannon Towers and Infantry effective against them.