Threshold concepts are transformative, integrative, and provocative; understanding these difficult concepts allows students to be capable of solving advanced problems. This investigation and evaluation of a metacognitive curricular approach explore variation in students' and teachers' discernment of structural complexity of concepts and its potential for enhancing students' learning and conceptual understanding of threshold concepts. Three trials of a metacognitive assessment activity administered to two cohorts of a civil engineering course (n?=?276 and n?=?264) were investigated. Students were presented with several answers (varying in structural complexity) to a question about a threshold concept and asked to mark each response. Quantitative analyses compared students' and teachers' marking schemes within and across trials, and qualitative analyses explored students' written reflections following the activity. Students' justifications for their marking schemes, their reflections on the activity's usefulness, and the convergence of students' and teachers' marking schemes suggest that the activity supported deep forms of student learning.