In War of the Monsters, players can choose from a number of monsters inspired by films like Godzilla, King Kong and other monster movies from the same era. Two monsters face each other in locations like a city or a power plant, and much of the environment can be used to attack. The fighting plays out in three dimensions, and the arenas are quite large, allowing combatants a lot of space to roam around. Large parts of the scenery are also destructible. Throwing another monster into a building will cause it to collapse.
The single-player mode follows each monster through a monster movie-inspired story, where the player will face both standard fighters and occasional bosses that require unique strategies. Unlike most fighting games, the action in single-player is seen from a wide, behind-the-back perspective, and the camera is freely adjustable. Completing the Adventure mode with each monster unlocks a short video that tells the monster's origin story. The unlockable characters, Raptros and Zorgulon, do not have origin videos.
The multiplayer mode in War of the Monsters supports only 2 players at once. Due to the size of the stages, the match is displayed in split screen unless the players are quite close to each other, in which case the two characters are shown from a side view.
In order to counter an alien invasion in the 1950s, scientists from around the world join forces in order to create a shockwave-producing weapon that short-circuits the flying saucers, causing them to crash. Unbeknownst to them, the saucers are powered by a radioactive liquid, which leaks from the debris of the ships and gives birth to giant monsters.
- Agamo: A tiki god made of stone and lava. Its special is a huge stone boulder.
- Congar: A giant gorilla driven by rage. Its special is a sonic roar.
- Kineticlops: Once a security guard, until a UFO crashed into a power plant and transformed him into a giant eyeball with limbs made of electricity. Electric waves are its special.
- Magmo: A four-armed rock monster. Its special rains lava and boulders onto opponents.
- Preytor: A giant praying mantis that resulted from an experiment gone awry. It can fly and attack using its pincers. Its special launches a larva that attaches onto an enemy and slowly drains life from it.
- Raptros (unlockable): A typical Western dragon. Its special is fire breath.
- Robo-47: A military robot whose special is a huge missile launched out of its head.
- Togera : A Godzilla-like giant reptile. Breathes radiation on enemies as his special.
- Ultra V: A huge Japanese mecha. Its special launches its arms out, pulling enemies or pieces of the environment towards it.
- Zorgulon (unlockable): One of the aliens from the crashed UFOs, mutated to become larger than the rest. Its special calls a swarm of small UFOs to attack.
- Sweet Tooth: A skin for Agamo unlocked by having a saved file of Twisted Metal: Black on the active memory card. Sweet Tooth's special is the same as Agamo's, with a flaming clown head instead of a boulder.
Each monster has four different skins to unlock, the first two usually being simple palette swaps, while the third and fourth are often more substantial changes, such as a bee version of Preytor, a female version of Ultra V, and even a lizard version of Kineticlops. These skins do not change the characters' movesets or other properties, however