Monsters are necessary for completing levels and serve the purpose of keeping enemies away from the demon queen. Monsters can only be created at graves in Monster Castle: Level Pack or placed in glowing blue spots in Monster Castle Defense. The types of monsters the player can create depend on how much bones they have. Upon creating a monster and clicking on it, the player can either choose to sell (by the far right button), evolve (by the middle button), or increase the power of a monster (far left button) by double clicking the respective button.
There are eight types of monsters which can easily be divided into two types, offensive and defensive. Offensive monsters attack enemies either by close range attack or from far away. Defensive monsters impede enemies by stopping them, with the exception of cropes which provides extra bones upon killing a enemy and Mudmans which also harms a enemy and pushes them away. Often defensive monsters can improve battle by being placed in front of a offensive monster as enemies are slowed down allowing the offensive monster to attack more.
All monsters have a base stat that can be increased. Once increased the monster will preform their skills better or longer. A monster's skill will also be enhanced once the evolve by gain EXP. To get rid of monsters the player can either sell them for bones or let the monster be killed by kamikazes, though the second option is not always available to the player and can't be controlled.
XP, which is the short form for experience points, is gained by defeating enemies. The rate at which a monster gains XP varies from monster to monster as does filling up the XP bar beneath it. When the EXP bar is maxed out the monster can evolve. After evolving twice, the XP bar will disappear. However, in Monster Castle Defense the XP bar is not shown beneath a monster but is instead only visible when the player clicks on the monster. Also the XP bar will not go away even when the player has evolved a monster twice instead it will allow the monster to go up a level without being upgraded.
Once a monster gains enough XP by killing enemies to fill up a bar below them, a star will appear. By double clicking the button middle button below a monster the player can evolve the monster, however, in newer versions of Monster Castle: Level Pack, monsters evolve automatically. Once evolved the skill of the monster will be increased and the monsters form will change.
Monster can be evolved up to two times and after the second time the monster will gain no more EXP. However, in Monster Castle Defense monster that have already been fully evolved can evolve again but this only raises one of their stats. Monsters that are evolved will cost more to upgrade. Also the monster will sell for more then when not evolved.
By clicking the far right button and clicking it once again, the player can sell a monster and gain bones. The player can sell monsters at any time though the price will not be as much bones as need to create a monster. Monsters that have been upgraded will sell for more then monster which are not and likewise monster that have been evolved.
Monsters can be upgrade by clicking the far left button. Upon being upgraded the power of the monster will go up one point and it will do better. The price to upgrade the monster will go up each time it is upgraded. When a monster evolves it will be upgraded by one even if the player has not upgraded it.
Main article: Sprouts
Sprouts or plants are offensive monsters and they attack by firing yellow projectiles at a long range. Sprouts costs 40 bones to create and starts with a attack of three (in the Level Pack version) or a attack of two (in Monster Castle Defense). Once evolved a sprouts projectiles become bigger.
Main article: Bonys
Bonys are offensive monster that attacks at short range with a club. It costs 20 bones to create and has a attack of five. Once evolved its attacks become stronger and it can turn around to attack its enemy.
Main article: Slimes (Monster Castle)
Slimes are defensive monster that slows enemies down once they touch it. Slimes costs 50 bones and when it evolves it becomes bigger and slows down enemies more. In Monster Castle: Level Pack, slimes start of with a attack of three while in Monster Castle Defense, it slows down a enemy by one.
Main article: Demons
Demons are defensive monster that briefly stops enemies by changing into an alternate form. It costs 100 bones to create and when evolved it slows down enemies longer and the form it takes to make enemies slow down changes. Demons have a attack of of three.
Main article: Mudmans
Mudmans are defensive and offensive monster that pushes enemies back and also harms them. When evolved it pushes enemies back faster. It takes 150 bones to create and mudmans have a attack of five.
Main article: Corpes
Corpes is a defensive monster that does not attack or impede enemies. Instead Corpes gives extra bones upon defeating enemies. When evolved how much bones a Corpe gives upon defeating enemies increases. It costs 200 bones to create and has a stat of one in the level pack version or a stat of three in Monster Castle Defense.
Main article: Hives
Hives are offensive monster that attacks by firing homing bees. Unlike all other monsters, hives can only attack enemies that are above or below it. It cost 80 bones to create and when it evolves its range increases and the rate at which it sends out bees. Hives have a attack stat of five.
Main article: Dragons
Dragons are offensive monsters that attack at a short range. Once evolved their range slightly increases and the fire they attack with may freeze enemies. They take 100 bones to create and have a attack of five.