We have used nanocavities constructed from Mn adatoms on Ag(lll) to achieve a controlled modification of the electronic structure of the Ag surface. Local tunneling spectroscopy reveals an intriguing modification of the electronic structure of a single Mn adsorbate put into these cavities. Model calculations show that the changes result from the coupling between the Mn s level and surface electronic states of the substrate, which are strongly influenced by the size and geometry of the nanocavity. The dimension and geometry of the adatom array thus provide a degree of control over the induced changes.