Young massive stellar clusters have come increasingly into the focus of discussions about the origin of PeV cosmic rays. Recently, the High Energy Stereoscopic System (H.E.S.S.) observed high-energy $\gamma$-ray emission around the young massive stellar clusters Westerlund 1, characterised by an energy independent, ring-like shape slightly off-set from the cluster position. We investigate the origin of this emission by modelling hadronic and leptonic emission processes with the open GAMERA library, discussing particle acceleration sites and propagation effects. Our findings support a predominately leptonic origin of the emission and highlight how the cluster's radiative and mechanical feedback facilitates particle acceleration.