Monsters are units produced in the Hatchery using Goo. Once they are hatched, they will be stored in the Housings until you juice them using the Monster Juicer, command them to defend using the Monster Bunkers and/or deploy them using the Flinger. However, only Pokeys can be hatched in the Hatchery at the start of your game. You need to unlock other monsters using the Monster Locker for you to be able to produce them.
Monsters can also be upgraded at the Monster Academy to increase their speed, health and damage but may also increases Goo cost, Housing space and and production time. Some monsters are also capable of learning special abilities using the Monster Lab.
Once you already have a pack of monsters you can now decide on how you would use them. You may either use them to defend your base and/or to attack yards around your base. In attacking, you must consider the favorite target of your monsters which are divided into five (5) categories:
1. Anything (any building nearest to them)
2. Defensive Buildings (the nearest Defensive building they see)
3. Resource Buildings (the nearest Resource building they see)
4. Monster Allies (the nearest friendly monster that needs healing)
5. Walls (the nearest Block)
Players usually attack Wild Monster Tribes to gain more resources. But, you may also attack other players' yards if you think you can beat them or if you just want to. Before you can attack you must first have a Flinger to deploy your monsters and a Map Room to locate your target's yard.
Aside from attacking, monsters can also be use to defend your base. They are to be stored on Monster Bunkers where they wait for intruders like the wild monsters and other players' monsters for them to kill.