We report on results from a set of numerical simulations of isothermal compressible turbulence forced at an intermediate scale with grid resolutions up to 8192^2. At a moderate pumping rate, the total energy of a typical frictionless system, subject to periodic boundary conditions, exhibits linear growth characteristic of an inverse cascade followed---as in the incompressible case---by condensation. The energy capacity of the compressible condensate, however, is limited by shock dissipation. Effects of compressibility are discussed in both shaping the inverse energy cascade at moderate Mach numbers and mediating the energy condensate growth that involves the formation of strong shocks, connecting centers of large vortices and short-circuiting the energy cycle. In the saturated regime, the total energy exhibits strong irregular oscillations caused by metastability of the compressible condensate.