This paper presents a design project and its assessment in an undergraduate control theory course. In the project, students mathematically modeled the thermal dynamics of a glass incubator and its heat source. Based on these models, they designed a lag compensator to keep the incubator temperature in a safe range when the external temperature fluctuates. It is hypothesized that activities planned in this real-world project can facilitate the learning of difficult controller design via the frequency response method. Multiple facets of student learning were assessed, including; (1) a two-level objective assessment based on students' written reports; and (2) four-category student self-evaluation surveys. The former identifies how the project helps achieve ABET outcomes and how each of the four project activities support learning outcomes. Surveys assess students' factorial knowledge and their perception about the project. Assessment results demonstrate that this project-based-learning experience effectively aids students in grasping difficult frequency-domain concepts and design methods.