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The formation, properties, and evolution of massive stars remain subject toconsiderable uncertainty; impacting on fields as diverse as galactic feedbackand the nature of the progenitors of both electromagnetic and gravitationalwave transients. The clusters many such stars reside within provide a uniquelaboratory for addressing these issues, and in this work we provide acomprehensive stellar census of Westerlund 1 to underpin such efforts. 69 newmembers were identified via I-band spectroscopy, yielding a total clusterpopulation of 166 stars with initial masses of ~25Msun to ~50Msun, with moremassive stars already lost to supernova. The stellar population follows asmooth and continuous morphological sequence from late-O giant through to OBsupergiant. Subsequently, the progression bifurcates, with one branch yieldingmid- to late-B hypergiants and cool super-/hypergiants, and the other massiveblue stragglers, prior to a diverse population of H-depleted Wolf-Rayets. Asubstantial population of O-type stars with anomalously broad Paschen serieslines are seen, a property which we attribute to binarity. Binary interactionis clearly required to yield the uniquely rich cohort of hypergiants, whichincludes both mass-stripped primaries and rejuvenated secondaries/stellarmergers. As a consequence future observations of Wd1 and similar stellaraggregates hold out the prospect of characterising both single- and binary-evolutionary channels for massive stars and determining their relativecontributions. This in turn will permit the physical properties of such objectsat the point of core-collapse to be predicted; of direct relevance forunderstanding the formation of relativistic remnants such as the magnetarsassociated with Wd1 and other young massive clusters (Abridged).

A VLT/FLAMES survey for massive binaries in Westerlund 1: VII. Cluster census