We present a detailed joint experimental and theoretical investigation of the valence band electronic structure of single crystals of the model cuprate ${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{4}{\mathrm{Cl}}_{2}$. This oxychloride system possesses a ${\mathrm{Cu}}_{3}{\mathrm{O}}_{4}$ plane which can be regarded as a superposition of two subsystems: a ${\mathrm{Cu}}_{A}{\mathrm{O}}_{2}$ cuprate plane and an extra ${\mathrm{Cu}}_{B}$ site, and thus represents an ideal trial system for assessing the impact of deviations from the ${\mathrm{CuO}}_{2}$ plane stoichiometry upon the electronic structure in cuprate materials. From polarization-dependent, k-resolved photoemission measurements the dispersion relations and symmetry of a number of the observed valence band features are determined and compared with the results of band structure calculations carried out within the $\mathrm{L}\mathrm{S}\mathrm{D}\mathrm{A}+U$ formalism, which include a detailed analysis of the character and symmetry of the individual bands. Upon electron removal, the extra copper site makes its presence felt via the formation of a second Zhang-Rice singlet located on the ${\mathrm{Cu}}_{B}$ subsystem, in addition to that originating from the ${\mathrm{Cu}}_{A}{\mathrm{O}}_{2}$ subsystem. The main valence band edge at $\ensuremath{\sim}$2 eV binding energy is shown to be mainly due to bands involving combinations of O ${2p}_{x,y}$ orbitals, some of which exhibit essentially pure O $2p$ character. These low-lying oxygen bands are different in origin from the nonmixing O $2p$ states observed, for example, at $(\ensuremath{\pi}$,$\ensuremath{\pi})$ in ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$, as the orbital combination responsible for the latter is involved in ${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{4}{\mathrm{Cl}}_{2}$ in the formation of the Zhang-Rice singlet state on the ${\mathrm{Cu}}_{B}$ sublattice.