Happy Wars is a multiplayer online game, defined as a role-playing strategy game, although it also has a short story-driven single player campaign mode. The multiplayer mode consists of two teams of up to 15 players attempting to destroy the other team's castle while also defending their own.
The players choose from one of three classes: the warrior, the mage, and the cleric. There are also subclasses, although the only ones implemented at the moment is the Warrior-type Berserker as well as the Mage-type Zephyr and the Cleric-type Engineer.
Source: Wikipedia, "Happy Wars", available under the CC-BY-SA License.