Treating cities as laboratories and as learning and experimental objects has many advantages playing an important role in fostering and supporting the circular transition. A city is an accessible and inexhaustible resource for considering motivating challenges to those learning any subject, and its nearness and complexity make it ideal for university students. This chapter explains this concept and describes the method and results obtained by two different examples and a related workshop with groups of students of the European Project Semester. The first was a learning experiment on the inclusive city carried out during the academic year 2018/19, and the second was on the redesign of the Valencia bus station as a challenge during the 2020/2021 academic year, including the workshop as preparatory activity. In this chapter, the authors explain a reflexive-type activity that contributes to exploring the city as a complex environment collaborating through university activity in the necessary transition towards more sustainable cities.