Quick breakdown of each unit type and what they do:
Soldier - The bog-standard unit of the game, good for getting up close and personal with other melee fighters. You can never bring too many of them to the party (unless of course you need specialized units!)
Archers - Great at long-range combat but extremely vulnerable due to their low armor. Save these guys for when you need to take down an object or enemy that inflicts devastating, up-close damage.
Mages - You'll need to reach the twelfth mission before you unlock these fireball-flinging bad-guys, although they can be accessed earlier if you don't mind spending a very large amount of gold coins.
Ballistas - These devastating catapults can be used to blast through even the most hardened barricades and towers. These unlock after you've completed the fifteenth mission.