Monster Mayhem: Build and Battle for the Nintendo DS is an action-strategy game with a focus on monster creation and combat. Players design their own monstrous creations by choosing various body parts and abilities, then train and lead them into battle against other creatures. The game combines creative customization with real-time battles, as players must strategically use their monster's unique skills to defeat opponents and progress through levels. The blend of design and fight mechanics caters to both creative players and those who enjoy tactical combat challenges.