The evolution of electronic excitation spectra under deposition of Cs on the polar GaAs faces (100) and (111) with various surface reconstructions is experimentally studied by means of high-resolution electron energy loss spectroscopy (EELS). Similar to the Cs/GaAs(110) surface studied earlier, for all crystal faces and surface reconstructions wt: found two energy loss peaks in the band gap of GaAs at Cs coverages exceeding 0.5 monolayer (ML). The analysis of the loss spectra proves that the rise of the loss peaks is due to a phase transition at which dispersed isolated Cs adatoms condense into densely packed clusters, eventually merging in a two-dimensional overlayer with metallic-type excitations. A simple model of plasmon excitations in a metallic layer placed on a dielectric substrate explains qualitatively the essential features of EELS experiments.