Here we report on the properties of the star forming factory, in which the continuous creation of stars results into a highly concentrated, massive (globular cluster-like) stellar system. We show that under very general conditions a large-scale gravitational instability in the ISM, that triggers the collapse of a massive cloud, leads with the aid of a spontaneous first generation of massive stars, to a standing, small radius, cold and dense shell. Eventually, as more of the collapsing matter is processed and incorporated, the shell becomes gravitationally unstable and begins to fragment, allowing for the formation of new stars. The shell keeps its standing location thanks to a detailed balance established between the ram pressure from the collapsing cloud which, together with the gravitational force excerted on the shell by the forming cluster, act against the mechanical energy deposited by the collection of new stars. The model accounts for a full analysis of feedback, as well as for the mass spectrum of fragments that result from the continuous fragmentation of the standing shell. This matches well, at both ends of the spectrum, the properties of a universal IMF. Other properties of the modelled clusters are here stressed.