One of the more complex topics to teach to software engineering students is the conceptual modeling design, which has several concepts that students must learn in order to specify the structural, behavioral and interaction views of software systems. Learning the design of class diagrams is of paramount importance since these diagrams are used to guide concrete development tasks such as programming and software testing, and-consequently-to avoid defective software products. Applying novel teaching/learning techniques in this topic may help students to reduce the defects that are committed at the moment of designing a class diagram. One interesting technique is the use of serious games, due to the fact that they provide learning environments free of risks and pressure for students, allowing the students to know the topics that they must learn in a fun way. Serious games have been widely used in programming courses. Thus, we aim to investigate the feasibility to replicate this experience for conceptual modeling of class diagrams at software engineering courses. In this paper, we present a roleplaying game specially focused in the class diagram, which is called Classutopia. This serious game provides modeling challenges, comprehension and correction of diagrams with different complexity levels for learning conceptual modeling design.