The class of Warrior is Single Physical Attack which can be transferred to Knight and Archer with fast speed and wide attack range. Knight and Archer usually are used behind the map to kill the escaped enemies.
The class of Gunner is Physical AoE Attack with slow speed but strong killing ability. It is always used in front of the stage. It can be transferred to Engineer with stun effect.
The class of Sage is Magic AoE Attack with strong killing ability. It can be upgraded to Flame Tower which can burn the enemies in the range.
The class of Mage is Magic Physical Attack with fast speed and wide attack range. It can slow the attack speed of enemies and electrify them if upgraded to TD with Ice Attribute.