Using a minimal model based on the continuum theory of a two-dimensional hard-core/square-shoulder ensemble, we reinterpret the main features of cluster mesophases formed by colloids with soft shoulder-like repulsive interactions. We rederive the lattice spacing, the binding energy, and the phase diagram. We also extend the clustering criterion [Likos, C. N. et al. Phys. Rev. E, 2001, 63, 031206; Glaser, M. A. et al. EPL 2007, 78, 46004] to include the effect of the hard cores, which precludes the formation of clusters at small densities.