Using a minimal model based on the continuum theory of a 2D hard-core/square-shoulder ensemble, we reinterpret the main features of cluster mesophases formed by colloids with soft concave 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.