Smiley Arena is a single-player game that combines tower defense with direct, action-based combat. The player controls a smiley character on the battlefield, moving with WASD and aiming with the mouse, while placing and upgrading turrets across five lanes to stop incoming waves of enemies. Six classes are available, each with its own ability: Fire (burn damage), Ice (slowing and freezing), Shadow (invisibility and critical hits), Medic (regeneration and healing), Bomb (splash damage) and Speed (a high-speed dash). Defeated enemies yield currency, which is spent on building turrets and upgrading them through several tiers. Level-up choices are made while the game continues to run, without pausing. Boss enemies appear at regular intervals between the standard waves. The full version features endless waves, all six classes, the complete upgrade tree and an achievement system. A free browser-based demo covering the first nine waves and the first boss encounter is also available. The game supports English and German.